Third-party presidential candidates finally will have their own debate: at 8 p.m. Sunday at Columbia University in New York.

The debate, which will be announced today, will include at least three of the four third-party candidates – independent Ralph Nader, the Green Party’s Cynthia McKinney and the Constitution Party’s Chuck Baldwin. Libertarian Party nominee Bob Barr said he has a scheduling conflict, but debate organizers say he wanted to appear only with Nader. (Democratic nominee Barack Obama and Republican nominee John McCain are also invited.)

Nader and Barr are on the ballot in 45 states, while the Green Party is on 31 state ballots and the Constitution Party is on the ballot in 37 states. Nader and McKinney also are on the District of Columbia ballot.

Organizers say the debate is an important exercise in democracy, especially because the debates sponsored by the Commission on Presidential Debates (the last of which is tonight) exclude candidates scoring below 15 percent in national polls. Nader, the best known of the candidates, has an average of 2.5 percent in recent national polls, according to realclearpolitics.com, while Barr averages 1.5 percent.

Nader maintains that if he could get into the debates run by the Commission on Presidential Debates, his numbers would immediately climb because “two-thirds of the people don’t know we’re running.”

“It’s a Catch-22.”

Nader describes the debate commission as “a two-party dictatorial company that doesn’t want anybody else on the stage.” The commission, created in 1987, is a corporation headed by two former chairs of the Democratic and Republican parties.

The third-party debate will be streamed at www.thirdpartyticket.com and will be shown on C-SPAN.