Nader is threatening to run simply because he can. It's like a grudge thing.

Also on hand — not participating in the debate but taking questions afterward — was Ralph Nader, the Green Party presidential candidate in 2000 and 2004, who hasn't yet ruled out another run this year. Some Greens and other high-profile figures are urging him not to run and expressing concern that he's become a polarizing figure who could hurt the party. Nader addressed the issue of party unity by saying, "I have very little to offer about how to unite the Green Party internally."

But he told the Guardian that if powerful institutional forces collude to limit his or the Green Party nominee's access to the ballot, as he charges they did in 2004, he might run to highlight the need for greater political participation, saying, "I'll be deciding within the next month." Nader has sued the Democratic Party, the John Kerry–John Edwards campaign, the Service Employees International Union, and a number of law firms and political action committees for allegedly conspiring to prevent him from running for president in 2004.