Some will tell us this is a terrible thing for Romney. But is it really? After all, idiots vote, too -- and if Trump can bring whomever might (incredibly) value his endorsement along, more power to him. Politics is a game of addition, after all.
Does it make Romney look less "serious"? Perhaps -- but whatever Romney's shortcomings are, a lack of "seriousness" isn't among them. And those who are inclined to support his candidacy aren't suddenly going to turn to Gingrich (who himself has many of Trump's flaws) or Santorum because of it . . . it's not like Romney is endorsing Trump.
Where Trump might have been dangerous would have been if he'd decided to run as a third-party candidate, giving those dissatisfied with the President somewhere else to go. If accepting his endorsement graciously helps foreclose that possibility, go for it.