WASHINGTON — Democratic candidates across the country are opening a fierce offensive of negative advertisements against Republicans, using lawsuits, tax filings, reports from the Better Business Bureau and even divorce proceedings to try to discredit their opponents and save their Congressional majority.

Opposition research and attack advertising are used in almost every election, but these biting ads are coming far earlier than ever before, according to party strategists. The campaign has intensified in the last two weeks as early voting begins in several states and as vulnerable incumbents try to fight off an onslaught of influences by outside groups.

As they struggle to break through with economic messages, many Democrats are deploying the fruits of a yearlong investigation into the business and personal histories of Republican candidates in an effort to plant doubts about them and avoid having races become a national referendum on the performance of President Obama and his party.

In Ohio, Representative Betty Sutton calls her Republican rival, Tom Ganley, a “dishonest used-car salesman” who has been sued more than 400 times for fraud, discrimination, lying to customers about repairs, overcharging them and endangering their safety. She warns voters, “You’ve heard the old saying, buyer beware!”

In Arizona, Representative Harry E. Mitchell accused his opponent David Schweikert of being “a predatory real estate speculator who snatched up nearly 300 foreclosed homes, been cited for neglect and evicted a homeowner on the verge of saving his house, just to make a buck.”

In New York, Representative Michael Arcuri introduces his Republican challenger, Richard Hanna, as a millionaire who “got rich while his construction company overcharged taxpayers thousands, was sued three times for injuries caused by faulty construction and was cited 12 times for health and safety violations.”

Negative ads can be successful, whether or not they are fair and fully accurate, particularly if they lure an opponent into responding or if they define a political newcomer before he can define himself. But they also carry risks, especially in a year when voters are frustrated about the economy and impatient with politics as usual.

“Our strongest piece of opposition research on Democrats is their voting records,” said Representative Pete Sessions of Texas, chairman of the National Republican Congressional Committee. “While character assassination seems to be the strategy for Democrats this year, the American people are supporting Republican candidates because they are providing an alternative.”

A debate has broken out among some Democratic officials about the effectiveness — or wisdom — of running such pointedly negative advertisements with five weeks remaining in the campaign. But party strategists said candidates did not have the luxury of waiting until the final stretch to go negative, particularly if the goal is to localize the races.

“When you’re talking about whether an individual really belongs in Congress, you’re not talking about the national issues,” said Steve McMahon, a Democratic consultant who creates television advertising for several candidates in the party.

He added: “Anytime you get personal in campaign advertising, there’s always a risk that it goes too far to be credible or it backfires altogether. That risk is typically mitigated by research that’s done in advance to determine the correct tone and what will make the ad credible.”

So far, many Republican candidates are forcefully defending themselves but not taking the bait by starting their own personal offensives. A review of television advertisements presented since Labor Day showed that the Republicans were basing theirs almost entirely on the records of Democrats on health care, the economic stimulus package and the first vote the Democrats cast when Congress convened in 2009: for making Nancy Pelosi speaker of the House.

Even many of the critical Republican advertisements produced by the candidates or the party are done with a softer touch. (Outside groups, which are investing record amounts of money for a midterm election, do not always follow suit.)

In Texas, Republicans are trying to defeat Representative Chet Edwards, a moderate Democrat who remains likable to many residents in his district, which includes the Crawford ranch of former President George W. Bush. An advertisement suggests that Mr. Edwards has changed in Washington, with a narrator calmly saying, “After 20 years, he’s not independent.”

In North Dakota, Rick Berg, a Republican trying to unseat Representative Earl Pomeroy, a Democrat, decided to make his latest ad about negative ads. “Isn’t it time North Dakota has a congressman who will change Washington and not change the subject with a negative ad?” Mr. Berg asked.

The Democratic ads are like a prosecutor’s case, carrying no rebuttals or countervailing facts. Yet for all of the protestations that arise every election year about negative advertising, there is plenty of evidence that the attacks are effective, particularly against candidates without a deep connection to voters. This year, as Republicans work to gain at least the 39 seats they need to take control of the House, many of their nominees have never run for office and come from business backgrounds, which often include a trail of documents.

For more than a year, a large team of researchers at the Democratic Congressional Campaign Committee has been digging into the backgrounds of potential Republican candidates. It is political detective work, with every piece of a candidate’s past fair game, so long as it is a part of the public record. In some cases, aides have been sent to districts for a closer look as they compile thorough dossiers, which are ultimately approved by a team of lawyers before the information is turned into television commercials.

Some of the material is rooted in policy, including the “Fair Tax,” a national sales tax pledge signed by many Republicans, which Democrats say would raise sales taxes by 23 percent. Many of the findings, however, are not rooted in legislation or policy.

In Tennessee, a Republican candidate accused his Democratic challenger of unearthing a divorce proceeding containing an accusation about intimidating his wife with a gun. In South Dakota, a poor driving record became a part of the dialogue after it was revealed that the Republican nominee for Congress had received 20 speeding tickets and numerous citations for failing to appear in court, running stop signs and not making payments, which twice resulted in arrest warrants.

Representative Kevin McCarthy of California, who leads the Republican recruitment effort in the House, said he warned candidates to conduct their own research before deciding to run so they were not surprised by anything dug up by Democrats.

“It’s called winning ugly,” Mr. McCarthy said. “But a lot of time it backfires, because people in their communities know them.”

Jim Renacci, the Republican nominee in Ohio’s 16th Congressional District, has owned a car dealership, nursing homes, an arena football team and other businesses. Four years ago, he was assessed nearly $1.4 million in unpaid state taxes, interest and fees, which he ultimately paid. The issue has been replayed again and again in television advertisements from his opponent, Representative John Boccieri, and Democratic groups.

“There are lemons, and then there’s used-car millionaire Jim Renacci,” an announcer said in an advertisement last week, as images of an old rickety station wagon appeared on the screen before segueing to an unflattering photograph of Mr. Renacci.

In an interview, Mr. Renacci dismissed criticism about his business background, saying: “He’s going to go over my 30-year experience and try and find this little thing or that little thing. As far as I’m concerned, you always know you’re doing well when your opponent is attacking.”

Many wealthy Democratic patrons, who in the past have played major roles financing outside groups to help elect the party’s candidates, are largely sitting out these crucial midterm elections.

Democratic donors like George Soros, the bête noire of the right, and his fellow billionaire Peter B. Lewis, who each gave more than $20 million to Democratic-oriented groups in the 2004 election, appear to be holding back so far.

“Mr. Soros believes that he can be most effective by funding groups that promote progressive policy outcomes in areas such as health care, the environment and foreign policy,” said an adviser, Michael Vachon. “So he has opted to fund those activities.”

The absence of these Democratic megadonors is contributing to a huge disparity in spending between pro-Republican and pro-Democratic groups. The groups wield huge influence in many House and Senate races because they can take in contributions of unlimited size.

In the last week, Republican-leaning groups outspent their Democratic counterparts on television by more than seven to one on Senate races and nearly four to one on House races across the country, according to data from the Campaign Media Analysis Group, which tracks political advertising. The gap shows few signs of abating, even with the midterm election season in full swing.

The donors’ reluctance stems from a variety of factors, including pessimism about the party’s prospects in November, but also President Obama’s strong condemnations of this kind of independent activity, both during the 2008 campaign and after he was elected.

For Mr. Soros, who was also a big donor in 2006 and 2008, it is a matter of being more focused on pushing to get the policy outcomes he wants than on the electoral process, Mr. Vachon said.

Mr. Soros gave $5 million each last year to organizations supporting the health care overhaul and the climate change bill, Mr. Vachon said. He also contributed $1.25 million this year to America Votes, an umbrella organization for a variety of liberal-leaning groups, which focuses on the voter mobilization that Mr. Soros has supported over the years.

The attention of Mr. Lewis, chairman of Progressive Insurance, also appears to be elsewhere this year. Jennifer Frutchy, who advises Mr. Lewis on his philanthropy, said he was focused at the moment on “building progressive infrastructure and marijuana reform.”

“That’s just where his head is right now,” Ms. Frutchy said.

Many major donors, in fact, seem to be drawing a distinction between continuing to support left-leaning policy organizations and other institutions, and giving money to political groups focused on this election.

Labor unions are still promising to spend large sums of money backing Democrats. But they are not keeping up at this point with the flood of money going to Republican-leaning organizations. The landmark Supreme Court ruling in the Citizens United case this year that eased restrictions on corporate political spending has certainly benefited Republicans, but so has the political environment.

“We’re concerned about it,” said Tim Kaine, chairman of the Democratic National Committee, which is abiding by a pledge to restrict contributions from political action committees and lobbyists. “Would it be easier to take that institutional money? Sure it would be.”

If the imbalance in spending continues, it would mark a reversal from the past, when Democrats held the advantage when it came to utilizing third-party groups that could accept unrestricted donations.

Interviews with Democratic operatives, most of whom would speak only on the condition of anonymity because they were discussing the internal dynamics of these groups, provided a broader portrait of the struggles they are facing as they court megadonors.

For donors, there is certainly an element of fatigue from giving cycle after cycle, as well as an economic squeeze brought on by the recession, the operatives said. But some more ideological donors are also upset that the Obama administration has not been more aggressive in pushing a liberal agenda. Big donors from Wall Street, including hedge fund executives and investment bankers, are also angry at the administration.

It also appears, however, that Republicans have outmaneuvered their Democratic counterparts since the Citizens United decision. They have taken advantage of Democratic broadsides against the ruling, which have inevitably had an effect on the attitudes of Democratic donors.

Mr. Obama devoted one of his weekly radio addresses this month to the effect he said untamed special interests were having on the midterm election. “We can see for ourselves how destructive to our democracy this can become,” he said. “We see it in the flood of deceptive attack ads sponsored by special interests using front groups with misleading names.”

Several Democratic strategists said the White House’s denunciations had made entreaties to prospective donors trickier.

“You can complain about the rules, or you can respond to them and fight back against the people who welcome those rule changes,” said Craig Varoga, who heads up Patriot Majority, which has been supporting Senator Harry Reid in Nevada and has been one of the most active Democratic-leaning outside groups.

Belatedly, some additional Democratic third-party efforts are shaping up. An organization called Commonsense Ten is emerging as a conduit for large checks directed toward Senate races and recently went up on the air with television advertisements in Missouri and Washington State.

The group has been convening regularly with other Democratic-leaning groups — labor unions, women’s groups and others — to coordinate efforts. It has lined up financial commitments to also get involved in the Senate races in Colorado, Wisconsin, New Hampshire and Kentucky, Democratic operatives said. The group is also contemplating jumping into races in Illinois, Connecticut and Delaware.

Over all, though, the group is talking about spending, along with its partner organizations, about $5 million, with commitments from donors so far for about half of that.

In contrast, American Crossroads and its affiliate, Crossroads GPS, the biggest Republican-oriented group involved in Senate races, has said it is well on its way to raising $50 million for this election.

Another group, America’s Families First Action Fund, is shaping up as a major player on the House side among Democrats. But it has not yet even broadcast any ads.

Democratic leaders have been increasingly sounding the alarm about the disparity in spending, which may be contributing to something of a thaw among donors, or at least more openness to these efforts.

“It’s a challenging fund-raising environment, to be sure,” said Jim Jordan, a veteran political strategist who is working with Commonsense Ten. “We’re seeing signs lately, though, of a growing sense of awareness and urgency and engagement among our donor base.”

Whether that will translate into a late flood of cash remains to be seen.