Will it matter and is this a surprise ? No on both counts. Everyone knows the establishment is horrified at the prospect of a Trump nomination and the notion of the party's last loser weighing in won't change things.
For those inclined to support Mr. Trump, a Romney backing of Marco Rubio only amplifies their rejection of old ways.
Senator Rubio has had lots of endorsements, with little to show for it.
Exclusive: Mitt Romney To Endorse Marco Rubio