WASHINGTON – Senator McCain and others pushing for tougher campaign-finance rules won a surprising ally yesterday as they announced a renewed effort to block partisan interest groups from spending millions of dollars in big donations in congressional and presidential races.

Senator Lott, chairman of the Senate Rules Committee, said he voted against sweeping campaign fund-raising restrictions won by Mr. McCain, Senator Feingold, and others in 2002 but now agrees with the law’s supporters that further legislation is needed to stop groups from circumventing the new rules.

Mr. Lott called six- and seven-figure checks that independent political groups poured into last year’s presidential election “sewer money” that must be stopped. The Mississippi Republican said his committee would vote next month on legislation that would put strict new limits on fund-raising and spending by partisan tax-exempt groups active in congressional or presidential races.

The proliferation of new groups that spent millions in the presidential race was an unintended consequence of the 2002 law that banned national party committees and congressional and presidential candidates from accepting big checks, Mr. Lott said. Money that used to go to the political parties simply moved to other partisan groups, he said.

“It’s been used by both sides and it’s going to be abused by both sides,” Mr. Lott said at a news conference with the bill’s sponsors.

The legislation proposed by Mr. Mc-Cain, a Republican of Arizona; Mr. Feingold, a Democrat of Wisconsin; Rep. Christopher Shays, a Republican of Connecticut, and Rep. Martin Meehan, a Democrat of Massachusetts, targets partisan tax-exempt groups known as 527s because of the section of the tax code that covers them. Senator Schumer of New York also said yesterday he supported the proposal, and Mr. McCain said President Bush does as well.

The bill must win approval from Mr. Lott’s committee before the Senate can vote on it. In the House, Speaker Dennis Hastert, a Republican of Illinois, has expressed interest in limiting 527 groups’ activities but hasn’t endorsed a specific proposal, Mr. Shays said.

Under the measure introduced yesterday, 527s that raise more than $25,000 a year and spend in congressional or presidential elections:

* Would have to register with the Federal Election Commission as political action committees, disclose details on their fund-raising and spending, and abide by the same contribution and spending limits as other PACs. That means they could only accept donations of up to $5,000 each from individuals or other PACs for their federal election activities, contributions known as hard money.

* Could accept up to $25,000 from each individual donor to help finance efforts that affect both federal and nonfederal candidates, such as get-out-the-vote drives or ads. They would be barred from accepting money from corporations or labor unions for such spending, and would have to cover at least half the cost with the smaller “hard money” contributions.

The new proposal is similar to one made in the last Congress. It comes as a judge considers lawsuits by Mr. Shays, Mr. Meehan, and the Bush campaign seeking to force the Federal Election Commission to use existing laws to limit activities of 527s.