President Bush signed the campaign finance reform bill into law in the Oval Office Wednesday without fanfare, but opponents of the measures immediately made hay of it by filing suit declaring the bill unconstitutional.

"I believe that this legislation, although far from perfect, will improve the current financing system for federal campaigns," Bush said in a written statement, pointing out several flaws in the bill.

"I believe individual freedom to participate in elections should be expanded, not diminished; and when individual freedoms are restricted, questions arise under the First Amendment. I also have reservations about the constitutionality of the broad ban on issue advertising... I expect that the courts will resolve these legitimate legal questions as appropriate under the law," he said.

The bill marks the first overhaul of campaign finance laws since the post-Watergate era.

The bill, which experienced several iterations since first being pitched in 1995, bans soft-money donations to national political parties, raises individual donation limits from $1,000 to $2,000, and restricts television advertisements paid for by special interest groups in the final days of an election season.

In South Carolina to attend a fund-raiser and speak to firefighters, Bush responded to questions about his reluctance to sign the bill.

"No I wouldn't have signed it if I was really unhappy with it. I think it improves the system. And it improves the system because it enables an individual to give more money. And what I want to do is have a system that encourages more individual participation, as well as more disclosure," he said.

Press Secretary Ari Fleischer and an Oval Office secretary were the only witnesses to the Oval Office signing. The president will send letters to the four sponsors of the bill, Sens. John McCain, R-Ariz., Russ Feingold, D-Wis., and Reps. Christopher Shays, R-Conn., and Martin Meehan, D-Mass.

Sen. Feingold responded to the news Wednesday, and said he was "thrilled" the president signed the bill.

"This is truly a historic moment, the first significant campaign finance reform in nearly 30 years. By ending the corrupt soft money system, this bill will begin to return the legislative process to all citizens, and help restore the public's confidence in their elected leaders," he said.

Immediately filing suit against the bill was Sen. Mitch McConnell, R-Ky., who last week named a legal team headed by former Independent Counsel Kenneth Starr to challenge parts of the bill, which McConnell has opposed from the beginning of the campaign finance debate.

He will be joined by a range of co-plaintiffs from the National Rifle Association to the American Civil Liberties Union.

"Today, I filed suit to defend the First Amendment right of all Americans to be able to fully participate in the political process," said McConnell. "I look forward to being joined by a strong group of co-plaintiffs in the very near future."

McConnell's legal team consists of Starr; well-known First Amendment lawyer Floyd Abrams; First Amendment scholar and dean of Stanford University Law School Kathleen Sullivan; general counsel for the Madison Center for Free Speech, James Bopp; and prominent Washington election lawyer Jan Baran.

The bill will not go into effect until after the November election.

After giving his signature, the president departed on Air Force One to speak with first responders in South Carolina and Georgia before heading to two fund-raisers expected to gather $2.5 million for Republican candidates Rep. Lindsey Graham, who is running for the Senate seat being vacated by Republican Sen. Strom Thurmond of South Carolina; and Rep. Saxby Chambliss, who is challenging the Democratic Senator from Georgia, Max Cleland.

Fox News' David Shott contributed to this report.