In short, they're "resisting" an agenda that is not theirs. While Trump loves to get under their skin and therefore makes it easy for them to be especially adamant in their resistence, I suspect that they would've done this with any Republican.
Of course it’s the (R) they really hate, and make up whatever lie needed to justify it. McCain is their media darling because he also hates Trump, but while he was running against Obama he was a coward, a traitor, a terrible husband, an incompetent, too old, and of course a racist.