Max Boot isn't so puzzled after all, since he soon unravels said puzzlement:
I don’t see what’s holding [Sen. Bob] Corker back from simply endorsing [Tennessee's former governor, conservative Democrat Phil Bredesen, over wacky Rep. Marsha Blackburn], especially now that he is retiring. The same is true with other Republicans critical of Trump: They should be urging voters to support Democrats, because that is the only way to rein in a president who threatens the rule of law. But precious few do so, because of a pernicious combination of self-delusion and self-interest. (Emphasis mine.]
And that he explains as well, conceding that "openly challenging [Trump] is suicidal," given the Trump-loving base, and that Republican pols are still "fool[ing] themselves into thinking that they are serving the same old Republican Party that once stood for conservative ideas such as limited government, fiscal responsibility, international leadership, and free trade."
It's all rather straightforward, really. And can't be emphasized enough.