WASHINGTON, Jan. 23 - The Supreme Court, ruling on Monday in an important campaign finance case, opened the door to a new round of legal challenges to the limits Congress placed four years ago on election advertisements paid for by corporations and broadcast during the weeks before federal elections.

The court's opinion was surprising, coming only six days after the argument. It was unsigned, barely two pages long and unanimous.

It may, however, have considerable impact, given that two years ago the court itself appeared to foreclose further challenges to the "electioneering communications" part of the Bipartisan Campaign Reform Act of 2002. The court upheld the law, usually called McCain-Feingold after its Senate sponsors, in a 5-to-4 decision that considered multiple free-speech challenges to the statute "on its face" rather than in particular applications.

The court ruled on Monday that both the government and a special three-judge Federal District Court here had misinterpreted its earlier decision as foreclosing future challenges to the advertising restrictions as they applied to particular advertisements or corporate sponsors.

The justices vacated the lower court's opinion and ordered it to consider the merits of an anti-abortion group's argument that the statute, if applied to an advertisement the group sought to broadcast on Wisconsin television stations beginning in the summer of 2004, would violate the First Amendment rights to free speech and to petition the government.

In upholding the electioneering-communication provision "against a facial challenge in the 2003 decision," the justices said, "we did not purport to resolve future as-applied challenges."

The disposition of the case bought the justices, who are closely divided on campaign finance issues, some time during a period of transition, and might have represented a strategic choice that appealed to both sides. The same might be said of the court's unanimous opinion last week in an abortion case from New Hampshire.

Many people who heard the argument in the campaign case last week came away with the impression that given the imminent departure of Justice Sandra Day O'Connor, who might not have been able to cast a final vote, the decision could well have been a 4-to-4 deadlock.

Election law specialists said the likely impact of the decision, Wisconsin Right to Life Inc. v. the Federal Election Commission, No. 04-1581, would be to reopen the issue of how to administer the law. It invites new litigation, they said, over whether specific advertisements fall under the law's definition of those that may not be broadcast within 60 days of a federal general election with money from a corporation's treasury.

Under the definition, an "electioneering communication" need not use words like "vote for" or "vote against." Rather, the law covers advertisements that refer to a clearly identified candidate for a federal office and are "targeted to the relevant electorate." Corporations must pay for such advertisements with money raised through a political action committee, which is subject to strict limits and reporting requirements.

In its lawsuit, Wisconsin Right to Life described itself as a grass-roots lobbying organization and said the advertisement it wanted to broadcast was part of a constitutionally protected lobbying effort.

The advertisement urged Wisconsin residents to call their senators, Herb Kohl and Russell D. Feingold, both Democrats, and encourage them not to filibuster President Bush's judicial nominees. Mr. Feingold was seeking re-election at the time. Wisconsin Right to Life has a political action committee, but it had only $13,000 in its account at the time, and the group sought to use money from its general treasury.

Edward B. Foley, an election law specialist at Ohio State University, said in an interview that while the court's action on Monday appeared narrow and procedural, it was actually neither in its practical impact.

The Supreme Court was telling the district court to define both the scope of a constitutionally required exemption from the law and the standard for winning such an exemption, Professor Foley said. If the standard proves "unworkable and messy," he said, that might give the law's opponents on the court a basis for re-examining the 2003 precedent itself.

Richard L. Hasen, an election law expert at Loyola Law School in Los Angeles, said the decision on Monday created new uncertainty about the court's campaign finance jurisprudence. "It puts the Federal Election Commission and the courts back in the business of evaluating the subjective intent of each ad," he said, adding that the law's sponsors had hoped to avoid that possibility by writing a broad but clear definition.

James Bopp Jr., who argued the case for the anti-abortion group, said the lower court "must now confront the real merits of this case, namely, that there is no constitutional justification for prohibiting grass-roots lobbying about upcoming votes in Congress just because we are in an election season."