The Supreme Court curbed a landmark campaign law Monday that sought to block last-minute television ads from corporations and unions that name candidates in federal contests.

But lawmakers and legal observers on both sides of the issue focused instead on the law’s remaining prohibition against “soft money” to party organizations rather than on possible attempts to revive the advertising regulations.

Sen. Olympia J. Snowe, R-Maine, who drafted the advertising provision, called the decision a disservice to the deep-seated problem of sham ads.

“The court has regrettably taken a giant step backward by opening a significant loophole for those who wish to blur the lines between issue advocacy and campaigning in order to unduly influence our federal elections,” she said.

But Senate Minority Leader Mitch McConnell, R-Ky., a vocal critic of the law, said the decision in Federal Election Commission v. Wisconsin Right to Life corrected an obstacle to free speech.

“This decision is a victory for free speech and confirmation that grass-roots advocacy organizations have the same free speech rights as all Americans,” he said.

The decision — just four years after the court upheld another aspect of the 2002 Bipartisan Campaign Reform Act (PL 107-155) — demonstrated the change in the 5-4 majority since Justice Sandra Day O’Connor was replaced on the court by Justice Samuel A. Alito Jr.

“What you basically need to get more of these regulations upheld is a change in the Supreme Court,” said Rick Hasen, a professor at Loyola Law School of Los Angeles.

With O’Connor, the court had upheld a ban on “soft money” that corporations, unions and affluent individuals gave to parties for uses other than directly urging support for specific candidates.

But a lower-profile aspect of the law prohibited corporations and unions from paying for, from anonymous treasuries, broadcast ads that name a federal candidate within 30 days of a primary or 60 days of a general election. The alternative is for the corporation or union to form a political action committee and name its contributors.

Groups ranging from the American Civil Liberties Union to the National Rifle Association argued that the law was unconstitutional because it prevented them from lobbying federal officials with broadcast ads.

The dispute hinged on what constituted electioneering, which a previous case had defined as “magic words” urging voters to support a specific candidate.

The court’s ruling Monday found that Wisconsin Right to Life should have been allowed to run ads in 2004 about judicial filibusters while Democratic Sen. Russ Feingold of Wisconsin, an original architect of campaign finance reform, was running for re-election.

Chief Justice John G. Roberts Jr. wrote that an ad could be regulated as “electioneering communication” only if “the ad is susceptible of no reasonable interpretation other than as an appeal to vote for or against a specific candidate.”

“The First Amendment requires us to err on the side of protecting political speech rather than suppressing it,” Roberts said.

In the dissenting opinion, Justice David H. Souter wrote that the decision “effectively, and unjustifiably” overruled the previous decision in the case.

Feingold suggested that the ruling could reopen the door to “phony issue ads and unlimited corporate and union spending on campaigns. . . . If that is the result, the court will have done the country a great disservice.”

The decision is expected to spur greater spending on a 2008 presidential campaign that has already generated the most intense fundraising in history, according to the nonpartisan Center for Responsive Politics. Candidates raised $150 million during the first three months of this year, compared with $880 million in total for the entire 2004 election cycle.

“This decision will mark the return of millions of dollars of unregulated special- interest money funding sham political ads,” said Bob Edgar, president of Common Cause.

Fred Wertheimer, president of Democracy 21, said it was important to keep in mind that the law’s prohibition against soft money is still intact.

“Whether the door has been opened partially or completely remains to be seen,” Wertheimer said of advertising.

Both supporters and opponents of the campaign law predicted that the decision served as an invitation for legal challenges to overturn other portions of it.

“This decision represents a severe blow to a campaign finance provision that amounts to nothing more than censorship,” said Jay Sekulow, chief counsel of the American Center for Law & Justice, an advocacy group that submitted a brief supporting the Wisconsin group. “This decision opens the door to further challenges of these free speech restrictions and represents a First Amendment victory for those who want to express their views on issues that matter most prior to an election.”

Moving any legislation is difficult in a Senate divided among 49 Democrats, 49 Republicans and two independents. But moving additional campaign legislation could prove even harder because McConnell led the challenge against the soft-money ban.

McConnell, who also filed a brief supporting the Wisconsin group, welcomed Monday’s decision as a way to “undo the stranglehold that campaign finance legislation has placed on political debate.”

Hasen was pessimistic that any other restrictions could be approved.

“I can’t imagine a campaign finance bill that could get through this Senate,” he said. “Forget it.”

Keith Perine contributed to this story.