He was against Trump from Day 1.
I happen to think; as you know I opposed the Trump candidacy. I don't think he is very well fit for the presidency. But fitness is not a reason for impeachment, and removal. High crimes are. Here we have a prosecutor looking for high crimes.
https://www.realclearpolitics.com/video/2017/08/04/krauthammer_removing_trump_from_office_would_be_a_catastrophic_mistake.html