ST. LOUIS -- Ralph Nader was barred from the presidential debate here Tuesday, hours after he sued the commission organizing the debates because he was excluded from the first one.

"Mark my words, this is the debate commission's last hurrah," Nader said after police turned him away from Washington University where Al Gore and George W. Bush appeared for the third and final debate. "Its power will be broken."

Nader, the longtime consumer advocate and Green Party candidate, was invited to do an interview with campus television station WUTV and had a credential from the station to speak with two reporters at their tent outside the field house where the debate took place.

But the university's police chief said the credential was invalid and Nader was turned away, even though two of his campaign staffers were allowed inside with the same type of pass.

Nader left the campus and his campaign aides said he planned to go to his hotel to watch the debate.

Earlier, Nader filed a lawsuit in Boston -- the site of the first presidential debate Oct. 3. He was denied access to the event even though he had been given a ticket by a local college student, and claimed his rights were violated.

That event prompted the lawsuit in U.S. District Court in Boston against the Commission on Presidential Debates, the commission's two co-chairmen, a commission "security consultant" and a State Police sergeant.

Nader called the lawsuit the first step in dismantling the commission.

"By the time I'm finished with the debate commission, its ranking in political opinion polls will be below the ranking of used car dealers," Nader said.