Washington ? Taking up the most significant campaign finance case in a generation, several Supreme Court justices Monday appeared skeptical of key provisions in a wide-ranging federal law that limits big-money contributions from corporations, labor unions and wealthy individuals.

Returning from their summer break a month early, the justices vigorously questioned lawyers on both sides about whether Congress had run afoul of the Constitution when it passed the Bipartisan Campaign Reform Act of 2002. The law, often called “McCain-Feingold,” after its main sponsors, Sens. John McCain, R-Ariz., and Russell Feingold, D-Wis., is under attack by other senators, political parties, unions, public interest groups and corporations.

At stake is a system that has funneled hundreds of millions of dollars into the electoral process. With so much on the line, the courtroom Monday was packed with senators, including McCain and Feingold, political party operatives and lawyers drawn by the wide range of constitutional issues in the case.

At the end of the session, however, few were predicting how the court would rule. A decision is expected by the end of the year, in order to provide guidance for the 2004 presidential campaigns.

Attorneys supporting the law told the justices it closed loopholes that allowed a small number of corporate and individual donors to influence or appear to influence federal elections.

“People who gave these huge contributions did not care about where it went,” said attorney Seth Waxman, a former solicitor general in the Clinton administration. “They cared about what it bought them.”

Monday’s arguments focused on two key provisions in the law. The first provision, which consumed the morning session, bans political parties from accepting unlimited contributions from corporations, unions or individuals — the so-called “soft money” donations that total hundreds of millions of dollars in an election cycle.

The second key provision, which the justices considered in the afternoon session, blocks corporations and unions from spending money out of their general funds to broadcast advertisements near Election Day that mention a particular candidate and are designed to influence the outcome of the election.