You wouldn't expect to plop into your easy chair, turn on the tube and find a U.S. Senate candidate from Minnesota on Letterman or Leno -- unless he happens to be Al Franken.

Franken appeared on David Letterman's CBS "Late Show" Tuesday night for a lively conversation dominated by his ongoing campaign for the DFL Party endorsement to oppose Republican Sen. Norm Coleman.

At one point, Franken showed his recent campaign commercial featuring his fourth-grade teacher.

At another, Letterman -- alluding to the troubles of New York's former governor -- asked Franken whether there is "anything we need to know about you and escort services."

Franken paused and responded: "I think Eliot Spitzer showed very bad judgment. ... And I promise not to indulge in any judgment that bad."

Franken called his prospects for getting the DFL endorsement "very good" and noted that his "chief rival," attorney Mike Ciresi, recently dropped out of the race, although he still faced "another gentleman ... who I like very much" -- college Prof. Jack Nelson-Pallmeyer.

"Why did the guy drop out?" Letterman asked. "Whores?"

"I really don't know, but I don't think so," Franken said.

Asked whether his background as a comedian worked in his favor as a candidate, Franken said Minnesotans are taking his candidacy seriously and pointed to his union endorsements and successful fundraising.

"We've put together a very serious campaign, but I have to be myself, so ... it's helpful to be funny. Funny is a good thing," he said.