What do you think of people like Mitt Romney being so negative about Trump?

I have to say, at this point I find it shocking. Mitt ran for president, what if people from his own party were extremely negative about him at this point in the contest? His behavior seems to lack the golden rule.

I guess, Romney must really think Trump will be a big problem? Really dangerous for America? Or, do you think Romney is just jealous? Maybe he is being pushed by the party like a puppet? I can’t figure it out. I find it a little surprising at this point.