Returning to the Washington political scene for a command performance, Kenneth L. Starr, the former lead investigator in the Whitewater land deal and Monica Lewinsky case, will head up the legal challenge to the campaign finance reform law that passed the Senate Wednesday.

Sen. Mitch McConnell, who claims the law on its way to President Bush's desk is unconstitutional, will be the lead plaintiff in the case. Other corporations, unions, and interest groups will also join the suit, McConnell said.

"This is a mission to preserve the fundamental constitutional freedom of all Americans to fully participate in our democracy," said McConnell, R-Ky.

The Senate on Wednesday gave final passage to the most far-reaching campaign finance legislation in nearly 30 years.

It bans the hundreds of millions of dollars in unregulated "soft money" that corporations, unions and individuals give the national political parties, and restricts in the final days before an election the use of soft money for "issue ads" that name a candidate, often with the purpose of attacking him.

Bush said the bill is "flawed," but promised to sign it because he said it improves the system overall.

McConnell said opponents plan to file their lawsuit before a three-judge panel in U.S. District Court in Washington, D.C., with the expectation that it would move quickly to the Supreme Court.

"These are perilous waters into which the Republic has now sailed," Starr said at a news conference with McConnell. "The questions are grave, the questions are serious. It is now time for the courts to speak authoritatively to what the Congress has chosen to do."

McConnell's case has some precedence. In 1976, following the last major overhaul, the Supreme Court ruled in Buckley v. Valeo that Congress could set limits on contributions, but that limits on spending violated free speech rights.

McConnell's case will focus on the soft money used for "issue ads" that will be banned 30 days before a primary or 60 days before a general election, he said.

Senate Majority Leader Tom Daschle, D-S.D., said he voted for the issue ad provision because "we think it's a very important contribution to the overall new framework we're trying to create with this bill."

But he added there is a clause in the legislation to ensure that the rest of the bill is unaffected if one part of it is struck down in the courts.

The bill would take effect Nov. 6, the day after this year's congressional elections. McConnell said they would like to see action on their challenge before then.

McConnell has some other heavy hitters on his team, including First Amendment lawyer Floyd Abrams; Jim Bopp, general counsel for the James Madison Center for Free Speech; Bobby Burchfield, an election lawyer who was involved in the Buckley v. Valeo case; Jan Baran, a past chairman of the American Bar Association; and Kathleen Sullivan dean of Stanford University Law School.

The Associated Press contributed to this report.