H. Russell Potts Jr., the independent candidate for governor, made his final play on Thursday to get into Sunday's debate between Democrat Timothy M. Kaine and Republican Jerry W. Kilgore -- or stop it if he can't.

Having failed to meet the requirement for participation -- 15 percent of likely voters in two public polls -- Potts turned to the federal courts in a last-ditch effort to win his way into the third and final debate of the 2005 campaign.

"This is an artificial, 15 percent criteria," Potts said in an interview. "The whole arrogance and smugness of establishing this threshold, it's a discrimination issue and a fairness issue."

In a lawsuit filed Thursday afternoon in U.S. District Court in Charlottesville, Potts asked a judge for a temporary restraining order that would force organizers to either cancel the hour-long televised event or invite him onstage with the other candidates.

Potts argues that he is a serious candidate because he has collected 24,000 signatures to get on the ballot and because of the campaigning he has done since then. He says University of Virginia professor Larry Sabato, moderator of the debate, and Sabato's Center for Politics, the sponsor, are arbitrarily keeping him out.

"Exclusion of Russ Potts from the October 9 debate would cause him . . . substantial and irreparable harm," the suit argues, "in that the best possible opportunity to communicate with and seek the support of the largest possible number of voters would be provided to his opponents and denied to him."

A hearing has been scheduled for noon Friday.

Attorneys for Sabato, who had anticipated that Potts might sue to be included, said they were ready for the tactic and do not expect it to be successful.

"I think it flies in the face of Supreme Court precedent," said William G. Broaddus, a former Virginia attorney general who is representing Sabato and the Center for Politics.

Broaddus cited a 1998 U.S. Supreme Court case, Arkansas Educational Television Commission v. Forbes, in which the court held that an independent candidate for Congress did not have the right to participate in a debate sponsored by a public television station.

The Potts lawsuit is the latest potential roadblock to a debate that has, at times, seemed unlikely to happen.

Last week, Kilgore campaign officials demanded that Kaine sign a "no-use" agreement ensuring that snippets of the debate would not end up in campaign commercials. Kaine refused and dared Kilgore to back out of the debate.

But the dynamic shifted this week, when Sabato and representatives of the debate's other sponsor, NBC-12 TV in Richmond, sided with Kilgore on the issue. They said the debate would be canceled if Kaine refused to sign the agreement.

On Thursday morning, Kaine signed the agreement "under protest," according to a campaign aide.

That was fine with Sabato, who has moderated many of Virginia's high-profile debates over the years. "Students submit things to me under protest all the time, so that's okay," he responded. "Looks like we have a debate."

The face-off in Richmond is scheduled to start at 7 p.m. and last an hour. It will be broadcast live in many parts of the state, including on NewsChannel 8 and C-SPAN in Northern Virginia.

Kaine said Kilgore's insistence on a no-use agreement showed he was "fundamentally opposed to the principle of open government." But he said he agreed to the demand to preserve the debate.

"We weren't going to give him any reason to get out of the only statewide televised debate," Kaine said of Kilgore. Kaine had pushed for several more debates in the past year, but Kilgore resisted. Kaine also has agreed to debate Potts and has done so three times. Kilgore has refused to debate Potts.

In a statement, Kilgore campaign manager Ken Hutcheson said of Kaine: "His attempt at political gamesmanship has been laid bare, just as we look forward to unmasking his failed, liberal record for all Virginians to see on Sunday evening."

Independent candidate H. Russell Potts Jr., left, and Timothy M. Kaine (D) have appeared together, but Jerry W. Kilgore (R) has refused to debate Potts.