Donald Trump is the president who paid hush money to the porn star he committed adultery with.
While I'm not sure what evangelicals actually believe in, I'm pretty certain that they're not really outraged by "sinful" or "sacrilegious" behaviour as long as it is committed by someone whose voter registration says "Republican".
Actual church attendance by Evangelicals has been going down for a while now. In much of the country it's as much a social--and, yes, political--identity as a religious one at this point.
Talk to college-educated Evangelicals, and Evangelicals without a college education, and you will hear two very different stories about Trump.
I know people in both camps.
I wouldn't be surprised if Biden tries exploits this schism.