Chief Justice John Roberts expressed doubts Tuesday about legal restrictions on political ads by outside groups as the Supreme Court took up a new challenge to the McCain-Feingold campaign finance law.

Questioning Solicitor General Paul D. Clement, who was defending the law, Roberts raised a hypothetical case in which a group runs an issue ad every month. Does the ad, he asked, become illegal in the months before an election?

Clement responded that such a group could continue to run the ads if it used political action committee money to pay for them, or if it refrained from identifying a candidate by name.

But Justice Antonin Scalia said that would undercut the purpose of the ad, adding, "The point of an issue ad is to put pressure on an incumbent you want to vote your way."

At issue is a provision banning the use of corporate or union money for ads that identify federal candidates two months before a general election. The case involves a lawsuit by Wisconsin Right to Life, which was barred from broadcasting ads that mentioned Sen. Russ Feingold, D-Wis., during his 2004 re-election campaign.

In the first challenge to how the law was working in practice, the group in 2004 sought an injunction barring the Federal Election Commission from enforcing the provision against it. But the U.S. District Court in the District of Columbia denied the request. A month later, then-Chief Justice William Rehnquist declined the group's request to intervene.

Roberts suggested that the fact that the ad also mentioned the state's other senator — Democrat Herb Kohl, who was not up for re-election that year — buttressed the group's argument that the ad was meant to influence legislation, not the election.

Clement called that a "twofer" for Wisconsin Right to Life. He said most ads that run in the final weeks of a campaign are designed to influence the outcome of elections.

The McCain-Feingold restriction was aimed at forcing groups to use regulated PAC money to pay for issue ads that are widely seen as thinly veiled election commercials. But Wisconsin Right to Life says an exception should be made for "genuine issue ads" that constitute grass-roots lobbying.

The group's commercials urged people to call Feingold and Kohl and ask them to oppose Senate filibustering of President Bush's judicial selections. Feingold co-authored the campaign finance law with Sen. John McCain, R-Ariz.

The Wisconsin group, which opposes legalized abortion, got a skeptical response from some justices, who said the court had already settled the issue when it upheld the law on a 5-4 vote in December 2003. Scalia voted against the law in that case.

Justice Ruth Bader Ginsburg, noting that the group opposed Feingold's re-election, asked why it didn't run the ad after the election if the commercial was aimed solely at lobbying.

The group's lawyer, James Bopp Jr., responded that a lobbying group makes its own determination about how to best influence legislation.

Justice Stephen Breyer said that in writing the law, Congress concluded it was impossible to tell whether such commercials were genuine issue ads or sham ads aimed at influencing the election.

"You have a very good argument," Breyer told Bopp. "But it's an argument we just heard in that (2003) case. ... Or are you asking us to go back and undo what we did?"

Both Breyer and Ginsburg voted to uphold McCain-Feingold in 2003, along with Justices David Souter, John Paul Stevens and Sandra Day O'Connor, who is retiring. If Roberts votes against the restriction in question, that sets up the possibility of a 4-4 deadlock if O'Connor is off the bench by the time the court issues its new ruling.

In that case, the court could either affirm the U.S. District Court's ruling in 2004, or schedule new oral arguments with Samuel Alito on the bench. Alito is expected to win Senate confirmation to replace O'Connor this month.

The case is Wisconsin Right to Life v. Federal Election Commission, 04-1581.