Anyone following Donald Trump's tumultuous first year in the White House has to wonder why Republican lawmakers won't stop defending a guy who is nasty to friends and foes alike.

Trump has repeatedly ridiculed his Republican colleagues, from calling House Speaker Paul Ryan a loser, to mocking Senate Majority Leader Mitch McConnell's failure to repeal Obamacare, and even berating Jeff Sessions — the guy Trump picked to be his attorney general. So why do these lawmakers stick by him? 

Because they're willing to weather the abuse if it means they get to advance their political agendas, according to 'Fire and Fury' author Michael Wolff.

"Mitch McConnell early on said, 'He will sign anything we put in front of him,'" Wolff told Bill Maher last Friday on 'Real Time.' "So [Trump] just becomes — for everyone in some way — a useful fool."

But Trump's usefulness has its limits, Wolff warned. "Now, the problem with that is that he's the useful fool, but then he goes wacko and says something that destroys everybody's plans to use him."

We saw a major example of that come to pass last week when Trump's infamous 'shithole countries' remark led to the government shutdown that began Friday.

Check out Wolff's other thoughts on Trump, including an ongoing affair that the author couldn't reveal but hinted at in his book.