WASHINGTON — The Supreme Court on Wednesday upheld a federal law that restored copyright protection to works that had entered the public domain.

By a 6-to-2 vote, the justices rejected arguments based on the First Amendment and the Constitution’s copyright clause, saying that the public domain was not “a category of constitutional significance” and that copyright protections might be expanded even if they did not create incentives for new works to be created.

The case, Golan v. Holder, No. 10-545, considered a 1994 law enacted to carry out an international convention. The law applied mainly to works first published abroad from 1923 to 1989 that had earlier not been eligible for copyright protection under American law, including films by Alfred Hitchcock, books by C. S. Lewis and Virginia Woolf, symphonies by Prokofiev and Stravinsky and paintings by Picasso.

The precise number of affected works is unknown but “probably number in the millions,” Marybeth Peters, the United States register of copyrights, said in 1996.