Issa did this because he BARREELLLYY won re-election. He knows that attaching himself to Trump, especially on an issue like this, would probably guarantee his defeat in 2020.
But at the same time, this is what I think will lead to Trump's downfall. Eventually you're going to see more and more Republicans refusing to get in line with the White House's bullshyt rebuttals. You're going to see more of them demand Trump's tax returns; demanding an independent/special prosecutor etc. The GOP establishment likes having Trump in power right now because he is signing executive order after executive order for shyt that they probably wouldn't be willing to VOTE for. It gives them cover. But eventually he will become a liability.