In reply to john arran:
It's actually rather more complicated than that, given that one of the most effective interrogators of Trump has been Fox's Megan Kelly, who has scared him so much he did at one point refuse to be involved in a TV debate which she was moderating.
It is perfectly possible for Trump to benefit from Romney's attack. Trump positions himself, with considerable success, as a political outsider, so being attacked by a Republican establishment figure who also happened to have lost a Presidential election, helps bolster Trump's image in his own eyes and in the eyes of many of his supporters.
The Republican party does seem to be in a bit of a pickle. Clearly none of the party leadership likes Trump, but they seem powerless to do anything about him. It'll be very interesting to watch what they do if he does win the nomination.