WASHINGTON — In its first argument of the new term, the Supreme Court on Monday considered whether to give employers a powerful tool to bar class actions over workplace issues.

The court’s decision on the matter could affect some 25 million employment contracts. A ruling in favor of employers, Justice Stephen G. Breyer said, could cut out “the entire heart of the New Deal” and undo an understanding of labor relations with roots in the administration of President Franklin D. Roosevelt.

The case is the court’s latest attempt to determine how far companies can go in insisting that disputes be resolved in individual arbitrations rather than in court.

The Supreme Court ruled in earlier cases that companies doing business with consumers may require arbitration and forbid class actions in their contracts, which are often of the take-it-or-leave-it variety. Such contracts typically require two things: that disputes be resolved by arbitration and that claims be brought one by one. That makes it hard to pursue minor claims that affect many people, whether in class actions or in mass arbitration.