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!”