As I have said before, I don't think the Republicans would stop Trump from being impeached. But as it stands, short of declaring him mentally unfit, it is unlikely the Republicans will initiate impeachment proceedings. I think 2016 has them spooked, they don't understand what happened, or what is happening, and they're playing it safe by trying not to anger their voting base further.
And as they(voters) appear to have favored Trump, they're keeping their hands off. Now come November, should the Democrats take the House, and certainly so if they(Democrats) get a Senate Majority, then all bets are off. I am inclined to think there is going to be a very hard push to get Trump to resign in November regardless, we shall see just how "bloody" that effort turns out to be. If that op-ed doesn't trigger an earlier bloodletting.
It could be Pence, particularly as he is the safest person in the group. Trump cannot make him resign, the worst Trump can do is exclude him from the 2020 Presidential ticket.