ASHINGTON, Jan. 15 — The Supreme Court today upheld the 20-year extension that Congress granted to all existing copyrights in 1998, declaring that while the extension might have been bad policy, it fell clearly within Congress's constitutional authority.

The 7-to-2 decision came in the court's most closely watched intellectual property case in years, one with financial implications in the billions of dollars. A major victory for the Hollywood studios and other big corporate copyright holders that had lobbied strenuously for the extension, the ruling had the effect of keeping the original Mickey Mouse as well as other icons of mid-century American culture from slipping into the public domain.

Justice Ruth Bader Ginsburg's majority opinion methodically dissected and rejected the arguments that a coalition of Internet publishers and other users of noncopyrighted material had marshaled against the Copyright Term Extension Act. The dissenters were Justices John Paul Stevens and Stephen G. Breyer.

The named plaintiff in the case was Eric Eldred, who wanted to publish some Robert Frost poems. Other plaintiffs included a church choir director; an orchestral sheet music company; a company that restores old films; and Dover Publications, a publisher of books that have passed into the public domain.

Organized by a Stanford Law School professor, Lawrence Lessig, who argued the case before the court in October, the plaintiffs did not attack the duration Congress chose for new copyrights: the life of the creator plus 70 years for individual works and 95 years from publication for copyrights held by corporations.

Rather, they argued that retroactive application of the 20-year extension to existing copyrights was a giveaway that violated the sense if not the literal words of the Constitution's grant to Congress of authority to "promote the progress of science" by issuing copyrights for "limited times." Extending existing copyrights would not promote new creativity, the plaintiffs argued, and a duration that is virtually perpetual in effect violates the meaning of "limited times."

But Justice Ginsburg said that history refuted the plaintiffs' argument. Going back two centuries, she noted that every time that Congress extended the duration of copyrights, which began with a 14-year renewable term in 1790, it granted the new terms to existing copyrights as well as to new works. This practice reflected a Congressional judgment that all copyright holders should be "governed evenhandedly under the same regime," Justice Ginsburg said.

In any event, she said, "the wisdom of Congress's action, however, is not within our province to second- guess" because the Constitution itself gave Congress broad discretion and the court only a very limited role in the area of intellectual property.

"As we read the framers' instruction, the copyright clause empowers Congress to determine the intellectual property regimes that, over all, in that body's judgment, will serve the ends of the clause," she said, adding, "We are not at liberty to second-guess Congressional determinations and policy judgments of this order, however debatable or arguably unwise they may be."

Paying something of a back-handed compliment to the plaintiffs, Justice Ginsburg said that "beneath the facade of their inventive constitutional interpretation" they were basically arguing that "Congress pursued very bad policy."

Justice Breyer spent much of a 29-page dissenting opinion explaining how bad, in his view, the policy was. The extension's "practical effect is not to promote, but to inhibit, the progress of `science' — by which word the framers meant learning or knowledge," he said. And while the Constitution speaks of a grant of copyright to "authors," he continued, the effect of the extension "is to grant the extended term not to authors, but to their heirs, estates or corporate successors."

Noting that the majority appeared to find the statute at worst unwise, but not unconstitutional, he said: "Legal distinctions, however, are often matters of degree, and in this case the failings of degree are so serious that they amount to failings of constitutional kind." He added, "I cannot find any constitutionally legitimate, copyright-related way in which the statute will benefit the public."