Corker is retiring.
That's the only reason he'a going after Trump. Don't you think the vast majority of Republicans aren't already convinced Trump is a thug, a bully and an incompetent ignoramus? No, they continue to make excuses for him because they want his agenda passed, and they don't want to alienate his supporters, who might primary them out.
Corker's no hero, he's just another cynical opportunist.