Chief Justice John G. Roberts Jr. expressed doubts yesterday 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, Chief Justice 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? Mr. 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...