Trump’s response is yet to come. If it’s like his previous responses he will not respond with substance but will call Romney a presidential loser, will point to the fact that Romney solicited (I bet he'll use the word "begged") Trump’s endorsement four years ago, etc. etc.
He will not likely respond to Romney’s well laid out case substantively against Trump – like the fact that his trade “policy” will lead to a trade war which will cost American jobs, businesses and workers. It likely would lead to our economy tanking. His election alone likely would mess with world financial markets and would speed allies and foes alike to assess a new strategy toward the U.S. This is not the establishment talking, as Trump will claim, this is reality TV.
Is there time to derail him? Sure, but Romney’s speech likely will not affect Trump’s one-third of the GOP vote. But it may affect the other two-thirds and maybe, maybe will give some of the one-third pause in coming primaries.