HARRISBURG, Pa. — Ralph Nader's bid to be listed as an independent candidate on Pennsylvania's presidential ballot was rejected Monday by a judicial panel that said he had forfeited the right by accepting the nomination of the national Reform Party.

Pennsylvania law prohibits a person who is affiliated with a political party from running as an independent.

The three-judge Commonwealth Court panel dismissed Nader's argument that the ban applied only to Pennsylvania residents.

The ruling strikes a fresh blow against Nader's campaign, which has been swamped with challenges to its nominating petitions across the country and was cheered by supporters of Democrat John Kerry, who was seen as having the most to lose if Nader were on the Pennsylvania ballot.

Samuel Stretton, the Nader campaign's Pennsylvania attorney, said he planned to appeal to the state Supreme Court "unless they tell me not to."

"That's a very significant decision," he said. "It would essentially be the death knell of any independent candidate" who is recognized elsewhere by a third party.

The national Reform Party endorsed Nader in May, potentially providing him ballot access in at least seven states, including the battlegrounds of Florida and Michigan. His running mate, Peter Camejo, is registered as a member of the Green Party.

Separately, officials in Missouri said Monday that Nader won't be on the presidential ballot there.

Secretary of State Matt Blunt had said Aug. 18 that Nader supporters had failed to submit enough valid signatures to qualify for the ballot. The Nader campaign had said it was considering whether to fight the matter in court.

Friday was the deadline to appeal, and court officials said Monday that Nader had filed nothing in Cole County Circuit Court, the seat of state government.

Nader's status on the Pennsylvania ballot is important to both Kerry and President Bush as they compete for the state's 21 electoral votes — the fifth-largest prize in the Nov. 2 election — in what could be an extremely close race.

Democrat Al Gore carried Pennsylvania in 2000, while Bush won Missouri.