I think that Mitt Romney believes that Donald Trump would make a terrible president and does not represent the best interests of conservative Christians. As a result, Mitt Romney is likely to endorse any of the other Republican hopefuls who stand a chance at beating Trump, namely Ted Cruz and Marco Rubio.
Mitt Romney will eventually back a republican candidate but it appears that he is waiting for the outcome so as not to influence the race. That said, there is one caveat; he doesn't want Donald Trump to win. Here is where it gets confusing: there is a system in place where the candidates are able to acquire delegates from each state yet when the person winning is not the chosen one by the establishment Romney has no issues stepping in and trying to influence the race. It's hypocrisy all over the place.
Mitt Romney will likely back Marco Rubio because Rubio, like Romney, represents the entrenched political establishment. Romney will likely also back any other GOP selection, except Donald Trump, and perhaps Ted Cruz, both of who represent the anti-establishment. Especially, Trump, who is the clear political outsider of all the candidates.
Yes, Mitt Romney will back the Republican presidential candidates that are not Donald Trump because he believes that Trump's rhetoric is too divisive. Romney is a respected individual in the Republican party and he has voiced on social media that Donald Trump's words and actions don't represent the conservative movement in America. Romney will either support Marco Rubio or Ted Cruz instead of Donald Trump.