NEW YORK (AP) – "Late Show" host David Letterman treated John McCain's decision to cancel an appearance on his talk show more like a stupid human trick than the act of a statesman.

The Republican presidential candidate said he was halting his campaign activities on Wednesday, citing the need to deal with the nation's financial crisis, and called Letterman to drop out of the show's late-night lineup. On the air on Wednesday night, Letterman assailed McCain's rationale and, with prickly humour, questioned whether the nominee – now trailing in some polls – was in trouble.

"This doesn't smell right," Letterman said. "This is not the way a tested hero behaves. Somebody's putting something in his Metamucil."

McCain spokeswoman Nicole Wallace said yesterday that the campaign "felt this wasn't a night for comedy."

"We deeply regret offending Mr. Letterman, but our candidate's priority at this moment is to focus on this crisis," Wallace said on NBC's "Today" show.

Letterman called McCain "a true American hero" but told his viewers: "This is not the John McCain I know, by God. It makes me believe something is going haywire with the campaign."

Instead of suspending a campaign, Letterman said, a presidential candidate should go to Washington to deal with a crisis and let his running mate shoulder the burdens of politicking.

"That's what you do. You don't quit. ... Or is that really a good thing to do?" Letterman said, a reference to McCain's running mate, Alaska Gov. Sarah Palin. "What's the problem? Where is she? Why isn't she doing that?" he asked. Letterman later asked: "Are we suspending it because there's an economic crisis or because the poll numbers are sliding?" Making matters worse for McCain, his replacement was MSNBC's "Countdown" host Keith Olbermann, a constant critic of the Arizona senator.

McCain told the CBS show that he was immediately flying back to Washington, Letterman told his audience. Then Letterman showed a TV feed of McCain being made-up for an appearance on news anchor Katie Couric's "CBS Evening News''.

"Doesn't seem to be racing to the airport, does he?" Letterman said. "This just gets uglier and uglier."

As McCain spoke to Couric, Letterman shouted at the feed: "Hey, John, I've got a question. Do you need a ride to the airport?" Letterman later said: "We're told now that the senator has concluded his interview with Katie Couric and he's now on Rachael Ray's show making veal piccata. ... What are you going to do?"