"Passing this bill will likely have a profound impact on each of us for the rest of our time here," Senate Majority Leader Tom Daschle, D-S.D., said shortly before Wednesday's 60-40 vote that sent the bill to the president. Just what that impact will be may be unclear, he said, "but we know this: the status quo is not acceptable, and today it will end."
In a written statement, President Bush said he would sign the measure but called it "flawed in some areas."
Senior advisers reluctantly decided Wednesday that Mr. Bush has to stage a bill-signing ceremony because he cannot afford to be labeled an opponent of political reforms.
"The legislation makes some important progress on the timeliness of disclosure, individual contribution limits and banning 'soft money' from corporations and labor unions, but it does present some legitimate constitutional questions," the president said in his statement.
The legislation, which passed the House last month, bans corporations, unions and individuals from making unregulated "soft money" donations to the national parties. Those contributions, often in five- and six-figure amounts, have ballooned from zero in the 1980s to $86 million in 1992 and nearly $500 million for the 2000 election.
The bill doubles, to $2,000 per election, the regulated "hard money" donation that an individual can make to a candidate.
"With the stroke of the president's pen we will eliminate hundreds of millions of dollars of unregulated soft money that has caused Americans to question the integrity of their elected representatives," said Sen. John McCain, R-Ariz., who has defied his own party leadership and gained national prominence while championing the issue.
McCain teamed with Democrat Russ Feingold of Wisconsin in the Senate, while Republican Christopher Shays of Connecticut and Democrat Martin Meehan became partners in the House, to win acceptance of their legislation.
Momentum began to turn in their direction in recent years because of the fund-raising scandals during the Clinton administration, McCain's focus on the issue during his try for the 2000 Republican presidential nomination and last year's collapse of Enron, the energy trading company whose executives were prominent political donors.
Mr. Bush, who in the past opposed the soft money ban, stayed clear of the fierce congressional debate. But he grudgingly embraced the bill in the end, making clear he would accept whatever Congress eventually came up with. "Had he said anything different we wouldn't be here today," said Shays.
While Democrats were the main supporters, they couldn't have succeeded without sizable Republican support. Forty-one Republicans joined Democrats in the 240-189 House vote last month. Eleven Republicans and independent Sen. Jim Jeffords of Vermont, a former Republican, backed the bill Wednesday in the Senate.
But Republicans, led by Sen. Mitch McConnell of Kentucky, have also made clear they will move swiftly to challenge several major provisions in the bill on the grounds that they violate First Amendment free speech rights.
One key target will be language that bars corporations and unions, in the final 30 days before a primary or 60 days before a general election, from using soft money for broadcast ads that on the surface deal with political issues but which mention candidates, often with the intention of attacking them.
McConnell said the banning of soft money to parties, but not other groups, "raises serious equal protection problems." He said the legislation "seeks, quote literally, to eliminate any prominence for the role of political parties in the American elections."
He intends to be the lead plaintiff in a case that will be filed soon in federal court in the District of Columbia. That case will go directly to the Supreme Court.
"We realize that the next step is the court challenge," McCain said. "We are confident that we will prevail."
In 1976, two years after the last major overhaul of campaign spending law, the Supreme Court upheld limits on campaign contributions but struck down provisions dealing with spending limits, equating spending with free speech.
The proliferation of soft money and "issue ads" since the early 1990s refocused congressional attention on campaign finance. But in 1992 the former President Bush vetoed legislation that allowed for partial public financing. In 1994, the House and Senate failed to reach a compromise between their different bills.
In 1995 McCain and Feingold began working together. They met strong resistance in the Senate, controlled by Republicans until last June.
Shays and Meehan managed to pass their bill in the House twice in the late 1990s. During the past winter, they had to resort to the unusual tactic of a signature campaign among House members to force the GOP leadership to bring their bill to the floor.
Separate from the Senate vote, the House Ways and Means Committee on Wednesday approved a measure exempting independent political groups that concentrate on state and local political campaigns from reporting their financial activities to the IRS.
Supporters of the legislation affecting what are called Section 527 groups said it would ease unnecessary reporting duties. Opponents warned it would open a new loophole for groups to operate in secret.