Congress has used the Constitution's Commerce Clause to fight prostitution and domestic violence, to break monopolies and to combat segregation — but its biggest test could come over the Obama administration's claim that it can compel individuals to buy health insurance.

A federal judge earlier this month struck down part of the new health care law on the grounds that Congress had stretched the Commerce Clause too far. Other federal courts, though, have upheld the law, and its fate is certain to be decided eventually by the Supreme Court.

The battle is just the latest in a long line of showdowns over the clause. While some see it as a critical tool to give power to the most important pieces of legislation, others believe Congress has applied it too broadly...