ASHINGTON, Dec. 10 — By a narrow vote and with a broadly pragmatic rationale, the Supreme Court on Wednesday upheld all the major provisions of the campaign finance law that Congress passed last year to try to curb a flood of money into national politics that had all but swamped a previous generation's effort at regulation.

The 5-to-4 decision rejected a First Amendment challenge to the new law, the Bipartisan Campaign Reform Act, brought by a coalition of unlikely allies that included the National Rifle Association, the American Civil Liberties Union, the Republican National Committee and the A.F.L.-C.I.O.

The plaintiffs argued that the central provisions of the law, a ban on unlimited contributions to the political parties, or soft money, and restrictions on some campaign-season television advertising paid for by corporations and labor unions, violated their First Amendment rights of free speech and association.

But throughout 146 pages of three separate majority opinions, the court found the new restrictions amply justified by the political system's recent experience. The principal opinion, which Justices John Paul Stevens and Sandra Day O'Connor signed as authors and Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer supported, referred to "reams of disquieting evidence" that existing contribution limits had been widely circumvented, with corrosive effects.

The decision locks in place rules that were imposed on political campaigns last November, diminishing the importance of soft money contributions, and giving an advantage to candidates, like President Bush and Howard Dean, who are most adept at raising the smaller, regulated donations known as "hard money."

The analytical fulcrum for the majority, and the heart of the debate within the court, was defining the type of political corruption that Congress could properly address. The definition mattered because the court's 1976 decision in Buckley v. Valeo, which established the modern constitutional boundaries for campaign finance regulation, held that regulations impinging on First Amendment rights were justified in order to curb corruption or the appearance of corruption.

Anchored in the abuses of Watergate, those words had a concrete meaning in 1976. The new law's defenders urged the court to look at the more subtle effects of soft money and to bring those within the definition of corruption.

To the dissenters on Wednesday — Justices Anthony M. Kennedy, Antonin Scalia, Clarence Thomas and Chief Justice William H. Rehnquist — corruption meant trading votes for dollars, or something very close to that. They said that current evidence of such behavior was insufficient to justify the new regulations.

But the majority rejected that definition. "This crabbed view of corruption, and particularly of the appearance of corruption, ignores precedent, common sense, and the realities of political fund-raising exposed by the record in this litigation," Justices Stevens and O'Connor wrote. They insisted instead that the problem with big, unregulated contributions was preferential access and the influence that comes with it, a more amorphous definition but one they said was amply demonstrated in the voluminous record that the law's supporters compiled in the lower court.

"As the record demonstrates, it is the manner in which parties have sold access to federal candidates and officeholders that has given rise to the appearance of undue influence," Justices Stevens and O'Connor said, adding: "It was not unwarranted for Congress to conclude that the selling of access gives rise to the appearance of corruption."

The dissenting opinions provoked by this analysis were bitter and vivid. "If the Bill of Rights had intended an exception to the freedom of speech in order to combat this malign proclivity of the officeholder to agree with those who agree with him, and to speak more with his supporters than his opponents, it would surely have said so," Justice Scalia wrote.

"Apparently, winning in the marketplace of ideas is no longer a sign that the ultimate good has been reached by free trade in ideas," Justice Thomas wrote, adding: "It is now evidence of `corruption.' This conclusion is antithetical to everything for which the First Amendment stands."

Justice Kennedy's 59-page opinion served as the dissenters' principal voice, with the other three signing it in whole or part. "This new definition of corruption sweeps away all protections for speech that lie in its path," he wrote, adding that the decision "leaves us less free than before."

Because the law has been in effect since Nov. 6 of last year, the ruling in one sense merely preserved a status quo to which players in the political system were adjusting, with more or less pain.

But because few people expected that the law would survive intact, that adjustment had been highly contingent on what the Supreme Court would do. The statute itself remained under a cloud of doubt that the justices' argument three months ago did little to dispel, particularly when the court indicated that the vote was likely to be very close.

So while the court's decision Wednesday in McConnell v. Federal Election Commission, No. 02-1674, changed almost nothing about the regulatory system that has been in place for 13 months, it ushered in a stark new reality. Beyond that, the decision indicated that a majority of the court, albeit a narrow one, was less interested in abstract doctrine than in the practical considerations.

For example, the majority opinion noted at one point that "Congress is fully entitled to consider the real-world differences between political parties and interest groups" in subjecting the parties to more extensive regulation. The opinion also upheld a limitation on soft-money contributions to state party committees on the ground that it was a practical necessity to prevent circumvention of the national party limits.

Leading up to the decision, the main unknown element was how Justice O'Connor — a former Arizona state legislator, and the only member of the court to have run for elective office — would vote. While other justices had staked out fairly clear positions on campaign finance, she had not. Her position became even more important after the Sept. 8 argument, at which Chief Justice Rehnquist indicated that he was likely to repudiate his earlier support for restrictions on campaign contributions by corporations. That prospect, which did come to pass, made the court evenly divided, with Justice O'Connor in the middle.

The structure of the decision was unusual, with Justice O'Connor and Justice Stevens sharing the authorship of the principal majority opinion. That opinion upheld Title I and II of the Bipartisan Campaign Reform Act, the most important provisions.

Title I prohibits the national political parties and their committees from accepting or spending soft money, the large, unlimited contributions that corporations, labor unions, and individuals outside the "hard money" limits set by the law. (In another, unanimous, part of the decision, written by Chief Justice Rehnquist, the court rejected a challenge to the increase in the individual hard-money limit to $2,000 from $1,000, ruling that the plaintiffs did not have standing.)

While most of Title I was upheld by a 5-to-4 vote, the vote to uphold the ban on solicitation of soft money by candidates themselves was 7 to 2, with chief Justice Rehnquist and Justice Kennedy joining the majority.

Title II establishes a new category of "electioneering communications," television advertisements that refer to specific candidates for federal office and that are broadcast in the relevant market within 30 days before a primary or 60 days before a general election.

Corporations and labor unions may not pay for such advertisements from their general treasuries, but must use money from their political action committees, which themselves are subject to contribution limits. Although this provision was widely regarded as one of the more vulnerable parts of the law, the majority's endorsement of it was so brief — six out of 119 pages in the Stevens-O'Connor opinion — as to be almost perfunctory.

In Chief Justice Rehnquist's portion of the majority writing assignment, the court struck down a prohibition on contributions by children under the age of 18, finding that it "sweeps too broadly."

A majority opinion by Justice Breyer upheld record-keeping provisions imposed on broadcasters.

The day was a most unusual one for Seth P. Waxman, the former solicitor general, who along with other lawyers at the firm of Wilmer, Cutler & Pickering had defended the law on a pro bono basis for its chief Congressional sponsors. He was in the courtroom to present the first argument of the day, in a case to which the justices had appointed him to represent a Nebraska prison inmate, John J. Fellers. Mr. Waxman said later that he tried not to listen when the chief justice began announcing the campaign decision, so that a loss would not cast a pall over his argument. Only gradually did he realize he had won the case.