The future of public financing for American political campaigns is in grave doubt after the Supreme Court declared unconstitutional a legal provision that raises contribution limits for congressional candidates who face wealthy, self-funded opponents.

While yesterday’s 5-4 ruling against the so-called Millionaire’s Amendment did not directly affect the legality of public financing, the decision is likely to undermine the political viability of such efforts by leaving them largely unable to adjust to free-spending candidates. In addition, one leading expert on election law said the justices’ rationale could jeopardize all campaign finance regulation by endangering the ban on corporate and union gifts to federal campaigns.

The majority opinion, written by Justice Alito, concluded that the government had no business trying to level the financial playing field between rich candidates and those less well-to-do.

“Different candidates have different strengths. Some are wealthy; others have wealthy supporters who are willing to make large contributions. Some are celebrities. Some have the benefit of a well-known family name,” Justice Alito wrote, joined by Chief Justice Roberts and justices Kennedy, Scalia, and Thomas. “Leveling electoral opportunities means making and implementing judgments about which strengths should be permitted to contribute to the outcome of the election. The Constitution, however, confers upon voters, not the Congress, the power to choose the members of the House … and it is dangerous business to use the election laws to influence the voters’ choices.”

The legal challenge was brought by a wealthy businessman from the suburbs of Buffalo, John Davis, a Republican-turned-Democrat who spent more than $1 million of his own money to run for Congress in 2004. In 2006, he spent more than $2 million. Neither campaign was successful, but he’s vowing to spend $3 million on another bid this year.

The court’s dissenters said Mr. Davis had suffered no harm and never could because the provision in dispute put no limits on his ability to spend. “The Millionaire’s Amendment quiets no speech at all. On the contrary, it does no more than assist the opponent of a self-funding candidate in his attempt to make his voice heard,” Justice Stevens wrote, joined by justices Breyer, Ginsburg, and Souter. “Enhancing the speech of the millionaire’s opponent, far from contravening the First Amendment, actually advance its core principles. If only one candidate can make himself heard, the voter’s ability to make a choice is impaired.”

Justice Stevens said Congress was furthering “proper and weighty goals” when it passed the millionaire rule as part of the Bipartisan Campaign Reform Act of 2002, also known as the McCain-Feingold law. “Minimizing the effect of concentrated wealth on our political process, and the concomitant interest in addressing the dangers that attend the perception that political power can be purchased, are … sufficiently weighty objectives to justify significant Congressional action,” he wrote.

The dissent also rejected Mr. Davis’s argument that it was unfair to increase the contribution limits for his opponent to $6,900 while donors to his campaign were still capped at $2,300. “It blinks reality to contend that the millionaire candidate is situated identically to a nonmillionaire opponent, and Congress was under no obligation to indulge any such fiction,” Justice Stevens said.

An election law specialist, Richard Hasen of Loyola Law School, said the court’s decision would set back attempts to enact new, publicly financed election schemes. “When I get calls about people wanting to implement public financing at the state and local level, the concern always is, ‘What about millionaires and how do we get people to opt in?’ The answer has been to give them extra funds, but this opinion calls that strategy into serious question,” Mr. Hasen said.

However, an advocate for tighter campaign finance regulation, Paul Ryan of the Campaign Legal Center, said the ruling was not a death knell for all “trigger provisions” which kick in when a big spender or wealthy outsiders weigh into a race. “The court didn’t go that far,” Mr. Ryan said yesterday. “There remain strong arguments in favor of the constitutionality of public financing trigger provisions, notwithstanding today’s decision.”

An attorney who regularly challenges campaign finance laws, James Bopp Jr., was jubilant about the ruling. “Good day for the First Amendment,” he said. “This decision has profound significance that goes way beyond the provision at issue.”

Senator Feingold and the presumptive Republican nominee for president, Senator McCain, stressed in separate statements that the justices’ decision did not affect the ban on unlimited “soft money” donations to political parties, a provision they said was the “core” of the landmark 2002 legislation they sponsored. Both men noted the Millionaire’s Amendment was not part of the initial bill. Mr. McCain’s statement did not acknowledge that he was among those who supported the floor amendment that the Senate passed, 70-30, in 2001.

While yesterday’s decision did not gut McCain-Feingold, the ruling may undercut a key feature of federal election law since 1974: a ban on corporate and union spending on federal campaigns. “The only justifications for those laws are based on an idea of political equality,” Mr. Hasen said. “I think the writing is on the wall.”