Health insurers led by WellPoint Inc. are backing Republicans with campaign donations by an 8-to-1 margin, favoring the party that’s promised to repeal President Barack Obama’s health-care overhaul if it wins back Congress.

Indianapolis-based WellPoint, along with Coventry Health Care Inc. and Humana Inc., gave Republican candidates $315,000 from May through July, according to U.S. Federal Election Commission records. That compares with $41,000 given to Democrats by the three companies as the parties near November elections that will determine who controls the U.S. House and Senate next year.

While Republicans aren’t likely to win the large majorities necessary to override a presidential veto and repeal the health law Obama signed in March, they may be able to slow or stall its implementation, said James Morone, a political science professor at Brown University in Providence, R.I.. At the same time, the turn to strongly favor Republicans may anger Democrats who had been receptive to insurers’ concerns, he said.

“It surprises me that they would make such a definitive move,” Morone said in a telephone interview. Focusing donations heavily on Republicans threatens to unite Democrats on policy issues that have divided them, including how strictly to control insurers’ business decisions about rates, he said.

Some Democrats have painted the industry as the enemy of efforts to improve the health-care system. House Speaker Nancy Pelosi has called insurers “villains,” WellPoint head Angela Braly was hauled before Congress in February to explain what Chairman Henry Waxman called “a breathtaking increase” in customer premiums, and Obama warned the industry in June not to undermine the law’s implementation.

Many of the insurer campaign contributions are aimed at congressional Democrats’ top Republicans rivals. WellPoint, Humana and Aetna Inc. gave $10,000 to House Minority Leader John Boehner of Ohio, who would probably take Pelosi’s job as speaker if Republicans win the House. Representative Joe Barton of Texas, the top Republican on the Energy and Commerce Committee headed by Waxman, got $4,500 from WellPoint. Many of the other contributions are to Republican candidates trying to knock off vulnerable first-term Democrats.

Democrats passed Obama’s $1 trillion health overhaul in March, after a year of debate in which they blasted insurers’ rate increases, profits and executive pay. The law includes restrictions on how much insurers can vary premiums on the basis of a person’s age and health status, and a requirement that limits profits and administrative spending.

Republicans unanimously opposed the law, and have since offered legislative proposals on the House floor to change or repeal part of it.

The campaign funding numbers represent a shift for insurers, who previously gave to Republicans by only a 2-to-1 margin.

WellPoint favored Republicans the most among the companies. Not a single dollar of its $144,000 that went to federal candidates and political committees in July was given to a Democrat, the records show.

In the past three months, Louisville, Ky.-based Humana favored Republicans 3-1, after previously splitting donations evenly between Democrats and Republicans since the start of the two-year election cycle that ends in November.

In May, June and July, the political action committees for WellPoint, Humana and Coventry gave a combined $314,900 to Republicans, 7.6 times as much as the $41,000 those companies donated to Democrats, according to the records.

Aetna, based in Hartford, Conn., and Cigna in Philadelphia kept their allocations among the two parties largely the same as earlier in the two-year election cycle, favoring Republicans by about a two-to-one margin. Still, many of the Democrats they have backed are members of the House’s “Blue Dog” Caucus, self-described fiscal conservatives who negotiated against some provisions of the health-care bill. Of the $13,000 Aetna gave to Democrats in July, about half went to Blue Dogs.

WellPoint, Humana, Aetna, Cigna and UnitedHealth Group Inc. have also been considering a $20 million-plus campaign fund to reward friends and punish enemies in Congress, two people familiar with the matter said earlier this month.

That fund would target vulnerable Democrats who have spoken out against the industry, and would support candidates who are likely to argue for the industry’s positions during future debate on the health overhaul.

WellPoint spokeswoman Kristin Binns declined to comment, and Humana spokesman Tom Noland didn’t respond to a phone call and e-mails requesting commen

Information on UnitedHealth’s campaign contributions for July isn’t available because the company doesn’t file data monthly with the Federal Election Commission. Through June, the Minnetonka, Minnesota-based company gave 44 percent of its $284,200 to Republicans and 56 percent to Democrats. UnitedHealth spokesman Tyler Mason also didn’t return a call and e-mail requesting comment on Aug. 23.

Campaign giving by political action committees in health-services industries, which include insurers, has grown every election cycle since 1998, when it totaled $1.1 million to federal candidates, according to the Washington-based Center for Responsive Politics. It peaked in 2007-2008 at $4.2 million over two years, and so far this election cycle totals $3.5 million.