WASHINGTON  Senate Democrats on Wednesday defeated a bid by Republicans to repeal last year’s sweeping health care overhaul, as they successfully mounted a party-line defense of President Obama’s signature domestic policy achievement.

Challenges to the law will continue, however, on Capitol Hill and in the courts, with the United States Supreme Court ultimately expected to decide if the law is constitutional.

The vote was 47 to 51, with all Republicans voting unanimously for repeal but falling 13 votes short of the 60 needed to advance their proposal.

Lawmakers in both parties joined forces, however, to repeal a tax provision in the law that would impose a huge information-reporting requirement on small businesses. That vote was 81 to 17, with 34 Democrats and all 47 Republicans in favor.