The only event called off during McCain’s so-called campaign suspension was an appearance on The Late Show with David Letterman. It’s a decision that may come back to haunt him even more than Sarah Palin’s train-wreck interview with Katie Couric.

In the world of late night TV, canceling a booking at the last minute is a cardinal sin. McCain had been a more frequent guest on the show than virtually any other politician and had developed the same friendly rapport with Letterman that he has cultivated with many of America’s mainstream media pundits. Not anymore. On the night McCain failed to appear, Letterman could barely contain his disgust , and he ended up being funnier than he’s been in months.

“You don’t suspend your campaign,” Letterman said. “No, because that makes me think, well, you know, maybe there will be other things down the road–if he’s in the White House, he might just suspend being president. I mean, we’ve got a guy like that now!”

But he didn’t stop there. “We’re suspending the campaign,” he said. “Suspending it because there’s an economic crisis, or because the poll numbers are sliding?”

Apparently McCain had called Letterman directly to cancel and told him he had to immediately fly to Washington to “save the country.” But Letterman found out that McCain was actually across town in another CBS studio having makeup applied for his interview with Couric. Letterman cut to the live feed exposing McCain for what he is–a liar.

It was more than great television, it managed to show what comedians can do best–cut through the bullshit.

Perhaps Letterman was just having a hissy fit. Or maybe he thought he’d seized onto a new bit he could exploit like his faux feud with Oprah a few years back. Whatever his motives, he tapped into a narrative that pundits have been skirting around for weeks. “This smells,” Letterman said, suggesting that McCain was simply not putting “country first.” He continued to rip into the Republican the next night, too. McCain’s “maverick” moves had grown so erratic by week’s end that McCain was becoming a comedic gift that kept on giving.

Letterman’s ire drew so much coverage that the McCain campaign was forced to address it, claiming they canceled the Letterman interview because they felt an appearance on a comedy show was inappropriate in light of the economic crisis. But that excuse raises more questions than it answers. The crisis was occurring long before the Letterman show was booked, so why agree to it at all? And why did McCain lie to Letterman about his whereabouts?

What McCain and his handlers may have failed to see is that there is a danger in allowing yourself to become the dominant source of joke fodder for America’s clown princes. The early digs at McCain were not unlike the prevailing comedic tone leveled at Bush. They were light and insubstantial. McCain was treated much like Bob Dole was in 1996, with easy jokes about his age and grumpiness. But after The View took him to task, a new angle emerged–that of McCain as the shameless politician who will say or do almost anything to keep up with Barack Obama.

Good comedians know when they’ve found a rich source of material. With the pressures of producing a fresh program five days a week, they may mine a successful joke until it grows stale. This is dangerous for McCain. Post-debate polls show that even if mainstream pundits are divided on who won the first presidential debate, the public believes Obama did. Before the debate, Obama was beginning to open up a significant lead and post-debate tracking polls seem to indicate that lead is growing and getting close to double digits. A weekly assault of jokes from Letterman–not to mention Craig Ferguson, Conan O’Brien and Jay Leno–could feed that trend.

Then there’s Saturday Night Live, another show that has had a longstanding affection for John McCain. The show has had rocky reviews and ratings in recent years and yet now seems to have regained much of its stature and popularity through the strength of Tina Fey’s brilliant impersonation of Sarah Palin. Her most recent appearance, which essentially recreated Palin’s bizarre, incoherent interview with Couric, was just as biting as Letterman’s critique of McCain. It not only touched on Palin’s inexperience; it emphasized her odd demeanor, beliefs and overconfidence.

These comedy skits are especially effective given their ability to gratify the public’s need to see politicians exposed in a brutally honest way that is avoided by cable news analysts. Viewers trust Letterman, SNL and others to be non-partisan but also to be top-shelf lie detectors. Jon Stewart and the writers at The Daily Show have turned this kind of work into an art form, but they still reach a relatively small audience that is much more unabashedly liberal compared to the viewers of traditional network comedy offerings.

If Letterman, SNL and others continue to hammer Palin as an extremist airhead and John McCain as a desperate madman, those images could start to solidify in the heads of voters more accustomed to listening to these entertainers night after night than to politicians. (Letterman has built up a loyal audience for over twenty-five years and SNL has been on the air now for close to thirty-five. That’s impressive longevity by any standard.)

We now know that McCain’s trip to Washington was little more than a distraction, and by some accounts may have even slowed down the process of closing a bailout deal. Later it was revealed that when an agreement was finally reached, McCain had been holed up in his office and at his expensive DC hotel. Some traditional media pundits have dared to point this out, albeit subtly. Does anyone think Letterman will handle McCain with kid gloves on this matter? Don’t count on it.

These are crucial final weeks for both campaigns, but McCain has a lot more to be nervous about. The foreign policy debate was supposed to be his to lose, and by most accounts he lost it. The next debates are going to focus primarily on domestic policy, and Sarah Palin’s recent appearances should be a serious cause for concern. Just as jokes about “inventing the Internet” and windsurfing seemed to caricature Al Gore and John Kerry, a similar fate as a punch line could await McCain. Better that than the presidency.