Reichsminister Doktor Goebbels wrote:

O'Rourke tends to be ideologically consistent, not so much party-bound, and especially less bound to the political fortunes of individual candidates. Limbaugh had to "carry the water" for candidates he didn't like because part of his role is to drive electoral politics. So Limbaugh actually had to go on the radio touting Joe Peckerwood for state senate in Tennessee, and wave the Republican flag, even though he knew the guy was not faithful to whatever metric Limbaugh uses to evalutate candidates.
O'Rourke doesn't. There is praise and criticism of Republicans in his work, most of it framed around their adherence or lack thereof to the kind of small government/individual liberty principles that used to be the stamp of the Republican party. When he says he voted for Bush "because he's a Republican," he's saying that even if the president is a turd, he's preferable to the alternative, when viewed in light of O'Rourke's ideology.