Yesterday, Mitt Romney had an op-ed go up at the Washington Post in which he said what pretty much everyone except the President's most ardent supporters think. The President has bad character.
Romney said nothing out of the ordinary for political conversations. In fact, some of the very people most outraged in public about Romney are, in private, open to saying the same thing Romney said publicly. That, however, is the problem. The outrage is not over what Mitt Romney said, but rather that he said it. By being willing to say publicly what others say privately, he puts the others in difficult positions. So they have to respond.
There's also a bit of ridiculous court politics at play. The courtiers have to dance around and parade for the king, so to speak. The ones screaming the loudest about Romney are typically the ones who most agree with him, but also need the grace and favor of the President.
If anything, the ultimate outrage from the President's courtiers is this part:
I will act as I would with any president, in or out of my party: I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not. I do not intend to comment on every tweet or fault. But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.
Mitt Romney is claiming he will be his own man, which in Trump's Republican Party is unheard of. There's a bit of jealousy in the outrage. Again though, the reason for the overwhelmingly outrageous outraged reaction is not because of what Romney said, but because he said it.