WASHINGTON — Chief Justice Roberts’s stated philosophy of ruling narrowly on cases before the U.S. Supreme Court will face a test when the nine justices decide the fate of a major piece of campaign finance regulation.

During oral arguments yesterday, several justices appeared ready to rule that the law, officially called the Bipartisan Campaign Reform Act of 2002 but better known as McCain-Feingold, restricts political speech beyond what the First Amendment allows. The prospect of a ruling that salvages the act against those concerns seemed distant.

A decision to strike down a central provision of the act would affect the 2008 election cycle by allowing organizations such as labor unions and the National Rifle Association to spend combined hundreds of millions of dollars on political advertisements on television and radio.

The law in question criminalizes the expenditure of union and corporate dollars for “issue ads” that mention a federal candidate’s name — even if only to urge the electorate to contact the candidate — during a blackout period before federal elections.

In 2003, the court upheld the law, 5–4, but invited organizations to challenge the law by arguing that their ads were not intended to have any effect on an upcoming election. Part of the court appeared to want to define a standard for exempting some advertisements from the law. Justice Scalia led a push to strike down the law.

“Maybe we were wrong last time,” Justice Scalia told the solicitor general, Paul Clement, who defended the act.

That is a view almost certainly shared by Justice Thomas. Although he was silent during yesterday’s arguments, Justice Thomas wrote a dissent to the 2003 opinion calling the law “the most significant abridgement” of freedom of speech since the Civil War.

Before the act, only advertisements that explicitly endorsed or rejected a certain candidate were regulated. As a consequence, issue ads rose in prevalence, and began, especially in the late 1990s, to play a major role in federal campaigns. Perceiving the issue ads to be a loophole in the heavily regulated campaign finance system, Congress classified most issue ads as “electioneering communications,” subjecting them to regulations.

In its 2003 decision, the court accepted that most issue ads were aimed at affecting the outcome of the pending election. The rest, which the court called “genuine issue ads,” could also be outlawed because of Congress’s difficulty distinguishing between the two. The swing vote could fall to Justice Alito, who spoke little yesterday.

The 2003 decision found a defender yesterday in Justice Breyer, who said the court’s decision was aligned with a century’s worth of jurisprudence upholding regulations on campaign contributions. Justice Breyer, among the majority in that case, seemed exasperated to see the law challenged again.

“You have an argument,” he told the lawyer challenging the law, James Bopp Jr. “I’m not denying that. I understand it. But it’s sort of déjà vu all over again. We’ve heard it.”

The advertisements the court considered yesterday are from 2004 and come from a group called Wisconsin Right to Life. They urge voters to contact Senator Feingold, a Democrat, to voice their disapproval of an anticipated filibuster in the Senate. The group maintains that the ads were grassroots organizing about a specific issue unrelated to Mr. Feingold’s pending re-election.

But judging the ultimate intent or effect of the ad proved difficult, and the court came to little consensus on deciding what such a standard would be, or if a workable one even existed.

Furthermore, requiring groups such as Wisconsin Right to Life to come to court to argue their case before they can run an advertisement struck some justices as problematic.

“Do we usually place the burden,” Justice Roberts said, “on the challenger to prove that they’re allowed to speak, as opposed to the government to carry the burden that they can censor the speech?”

The case is called Federal Election Commission v. Wisconsin Right to Life.