Well the Republicans don't because his base has proven over and over again that nothing he does or says makes the slightest difference, they'll support him because at this point it's become a religious doctrine for them. And no amount of evidence or facts will make someone abandon a religious doctrine, they have to decide for themselves to change their minds. And what Party doesn't love a group of people who will
always do what you want them to do if you just play the right tune for them? As for the Democrats, they don't need his base, they need to get the "My vote wont matter!" crowd to vote for them, believe me, if they get those guys to vote, things will change in 2020.
"lesser"? They vote for/support him
because he's the greater evil. They LOVE his foul mouth, his name calling, his lying and his inability to keep it in his pants. If they didn't? They wouldn't have voted for him in the first place. They wouldn't have supported him with an undying, unswerving loyalty and they wouldn't be so eager to try and get him re-elected in 2020.
tulc(knows people may "vote for the lesser evil" but they don't KEEP voting and supporting it unless they've embraced that evil, no matter what excuse they have to tell themselves to justify their doing so)