WASHINGTON – Denouncing the nation’s campaign finance regulator as “arbitrary and capricious,” the congressional sponsors of federal campaign finance laws yesterday filed a lawsuit asking a federal judge to force the Federal Election Commission to write new rules reining in the independent political groups that have run harshly negative ads against the presidential candidates.

The suit is the latest strike in an ongoing offensive by advocates of strict campaign finance laws against the commission, which – after lengthy consultation and delay – voted last month against subjecting the well funded independent political groups to the same strict fund-raising rules that apply to political committees.

“The FEC has become the biggest campaign finance problem we have today,” said the president of the campaign finance advocacy group, Democracy 21, Fred Wertheimer, who is a member of the legal team representing the plaintiffs.

The suit was filed in federal district court for the District of Columbia on behalf of Rep. Christopher Shays, a Republican of Connecticut, and Rep. Martin Meehan, a Democrat of Massachusetts, who had been the original House sponsors of the Bipartisan Campaign Reform Act of 2002.

Senator McCain of Arizona and Senator Feingold of Wisconsin, the principal Senate sponsors of the legislation also known as the McCain-Feingold law, filed a friend-of-the-court brief. Mr. McCain has said he plans to introduce legislation in the Senate next week that would impose new regulations on the so-called 527 groups.

The congressmen accuse the commission of engaging in a “massive evasion, circumvention, subversion, and violation” of campaign finance law by failing to prevent the 527 groups, named after a section of the tax code that governs them, from accepting unlimited contributions, some in the millions of dollars, to help promote and defeat the presidential candidates.

They also note the commission has yet to respond to specific complaints against individual groups, including the liberal-leaning group The Media Fund; the Swift Boat Veterans for Truth, who have criticized Senator Kerry’s war record, and the conservative-leaning Progress for America Voter Fund.

“The Commission should be required to issue rules to ensure that section 527 ‘political organizations’ are complying with the law and are not improperly spending tens of millions of dollars of soft money to influence federal elections,” the lawsuit says.

The chairman of the commission, Bradley A. Smith, said the lawsuit was “politics.”

“I think the commission’s bipartisan majority is comfortable that we got the law right. It’s hard for me to imagine the decision was arbitrary and capricious when the overwhelming majority of public comments also took the position adopted by the commission,” he said.

In August, four out of six commissioners voted against a proposed rule that would treat 527 groups as political committees if their “major purpose” was to influence federal elections.

Commissioner Ellen Weintraub, a Democrat, said she voted against the rule because Congress itself had not included the 527 groups in campaign finance laws.

“I don’t have any preference at all on the rule. I’m just trying to honor the choice Congress made. They can make other choices in the future, and I’ll be happy to enforce that law if they decide to change it,” she said.

The legal case is also unlikely to be resolved before November. The plaintiffs will have to overcome the wide berth of discretion that courts give to regulatory agencies.

“It’s a pretty hard kind of case to win, but not impossible,” said a law professor at the University of California at Los Angeles law school, Daniel H. Lowenstein, who specializes in election law.

The lawmakers argue that the commission is not entitled to deference because it has simply avoided its job of making a rule.

“This is a case where the agency itself has said it should issue regulations, yet they have failed to do that,” said the director of litigation for the Campaign Legal Center, Gerald Hebert, a member of Mr. McCain’s legal team.

The outcome could depend on the attitude of the judge hearing the case, Judge Emmet Sullivan, said Mr. Lowenstein. “The average judge would probably say, this is political and we shouldn’t get too deeply enmeshed in this. Another judge may say, the FEC was wrong,” he said.

The case comes as another federal district court judge, James Robertson, a Clinton appointee, holds a hearing today in a separate lawsuit filed earlier this month by President Bush’s campaign. The Bush campaign has asked for an injunction to force the commission to act within 30 days on complaints the campaign filed against several liberal 527 groups last March.