The lawmakers, in a statement, said their defense team was filing intervention papers in response to lawsuits brought by the National Rifle Association and Sen. Mitch McConnell, R-Ky., "to defend the constitutionality of all spects of the new campaign finance law."

The NRA and McConnell, a longtime foe of the legislation, filed lawsuits before a three-judge court in Washington just hours after President Bush signed the legislation into law March 27. They contend that the law's restrictions on the use of money in political campaigns violate First Amendment free speech rights.

The case is expected to move quickly to the Supreme Court.

McCain, R-Ariz., was joined by his partner in the Senate, Russ Feingold, D-Wis., and the two chief House sponsors of the legislation, Reps. Christopher Shays, R-Conn., and Martin Meehan, D-Mass. Also participating were Sens. Olympia Snowe, R-Maine, and James Jeffords, I-Vt., who sponsored the provision in the act that is a main target of the constitutional challenge.

The law bans corporations, unions and individuals from donating unregulated sums of "soft money" to the national political parties. The Snowe-Jeffords language in the measure also bars the use of soft money in the final 30 days of a primary or 60 days of a general election to broadcast "issue ads" that name a candidate, usually with the purpose of attacking him.

The papers filed by the McCain team state that they will show that "by closing loopholes in current law and prohibiting clearly identifiable abuses, the Reform Act encourages renewed citizen confidence and participation in all aspects of our democracy, thereby strengthening First Amendment values."

The supporters noted that the Justice Department, as it traditionally does in challenges to federal law, will take the lead in defending the measure that passed Congress last month after a seven-year struggle by the McCain-Feingold and Shays-Meehan teams.

McCain and the other lawmakers are to be represented by Seth Waxman, former solicitor general in the Clinton administration and currently a partner at Wilmer, Cutler & Pickering, and Burt Neuborne, the legal director of the Brennan Center for Justice at New York University School of Law.

Among McConnell's attorneys are former independent counsel Kenneth Starr and Floyd Abrams, a leading First Amendment lawyer.