“Late-night hosts used to pride themselves on their viewers not knowing which way they leaned politically,” says Johnson, who blogs about the intersection of popular media and Washington politics at Wilshire & Washington.

“What changed all of this was Jon Stewart and now the one-two punch of Jon Stewart and [Stephen] Colbert,” says Bob Thompson, the Director of Syracuse University’s Bleier Center for Television and Popular Culture. “Even though their numbers aren’t as high [as Letterman’s], they are the ones getting all the attention.”

“All of the late-night comedians have to in one way or another come to grips with that. Letterman did so by becoming more political and more focused and acerbic in his political humor, trying to move in the direction of Jon Stewart.”