UPDATE: Given the school shooting in Florida, I'm told the Romney team is going to delay the announcement.
There is only going to be marginal opposition to Romney. It looks like he is set for an easy walk into the Senate fro Utah. Whether he intends it or not, the media is going to do what they can to structure a Romney v. Trump situation. I hope Mitt Romney's advisors have figured out a way to help him navigate that and handle it.
It seems pretty clear to me that an obvious media and Democrat strategy is going to be to find issues where there is separate between Romney's views and Trump's views, then rush out to other Republicans to demand to know if they are with Team Mitt or Team Don. The whole process will be designed to fuel the "GOP Civil War" narrative the media has been pushing. It'll also be used to cause problems for Republicans in primaries where Trump support is most intense.
Surely the Romney Team is smart enough to know this is going to happen and have worked to address it. Otherwise, Romney is going to see a lot of Trump supporters blaming him for November's pending losses.