Continued from page 1

The law does not apply to books, Ms. Kagan said, but it might apply to pamphlets. Because it was unclear, she suggested, the topic was ripe for a future legal challenge.

That brought a rebuke from Justice Samuel A. Alito Jr., who questioned how anyone could be expected to know what types of electioneering a corporation or union could legally finance.

“In light of your retraction,” Justice Alito said, “I have no idea where the government would draw the line with respect to the medium that could be prohibited.”

Theodore B. Olson, who represented Citizens United, said Ms. Kagan’s acknowledgment demonstrated that the laws are so muddled that his client would have no way of knowing whether the disputed ads represented constitutionally protected speech or would leave the group open to prosecution.

Justice Sotomayor seemed almost immediately to be in the minority in her debut case.

In her first question from the bench, she raised an issue that has been central to campaign reformers that the court’s decision to consider a broader challenge to campaign-finance laws was abrupt, ignored long-standing precedents and ran afoul of the court’s normal deliberative process.

She asked First Amendment lawyer Floyd Abrams whether changing the law now would “do more harm than good.”

“Your honor, I don’t think you’d be doing more harm than good in vindicating the First Amendment rights here, which transcend that of Citizens United,” replied Mr. Abrams, who was representing Mr. McConnell.

Court watchers say the outcome will hinge on the opinions of Chief Justice Roberts and Justice Alito, whose views on campaign-finance law have not been clear in prior cases. Justice Alito replaced Justice Sandra Day O’Connor, who had served as the decisive fifth vote in fending off several challenges to the McCain-Feingold law.

But any thought that either justice could step in and save the law appeared to evaporate as they hammered away at the positions taken by government attorneys.

Ben Ginsberg, a veteran campaign lawyer, said after the session ended, “I think they are pretty unsympathetic to the notion of keeping the statute the way it is.”