"Late Show" viewers must be starting to wonder whether David Letterman hates old people or just John McCain.

Maybe more important, is the host having a hard time hiding his support for Barack Obama?

Take for example Thursday's opening monologue. As the topic changed to the presidential campaign, Letterman focused most of his comedic attacks on the GOP frontrunner and also-ran Ralph Nader while actually not offering one joke about Barack Obama.

For those interested, here was the joke tally from Thursday's monologue:

"I like how you think, senator," cooed "Late Show" host David Letterman in agreement with John Edwards's charge that "most of what" Fox News Channel host Bill O'Reilly says "is crap."

Letterman had asked the former senator about his "feud" with O'Reilly over Edwards's charge that the Bush administration is failing to care for military veterans to the extent that hundreds of thousands are winding up homeless.

The exchange came in a jovial January 22 interview in which Edwards joked about having Letterman as his running mate, or at the very least as a celebrity endorser a la Oprah Winfrey.

Video (1:04): Windows Media (1.96 MB) or MP3 audio (473 kB).:

If things don't work out for Hillary with this presidential thing, she can always do stand-up out in LA. Or not. If you didn't catch her side-splitter on last night's Letterman, you can view it here, as rebroadcast on MSNBC this morning.

For those taking nitrates who might not want to risk a sudden drop in blood pressure by watching the clip, here's the text of Hillary's rib tickler:

Dave has been off the air for eight long weeks because of the writers' strike. Tonight he's back. Oh well. All good things come to an end.

Hillary, stop. You're killing me here.

That Hillary Clinton -- what a card!

According to an email I've received from her campaign, the intrepid Hillary is venturing into the lion's den, with TV appearances scheduled tonight on Letterman and September 4 on the Ellen DeGeneres show. Daring stuff!

In any case, according to the email:

Ellen is soliciting questions from her viewers to ask of Hillary, and we want to turn it around on her. So if you have a question for Hillary to ask Ellen, submit it here.