Chris Cillizza calls it:
In just the last 9 days — since Romney won the Illinois primary — he has been endorsed by former Florida governor Jeb Bush and former President George H.W. Bush while leading tea party Senator Jim DeMint has said kind things about him and Freedomworks, a tea-party aligned group, has dropped its opposition to him as the nominee. Add Rubio’s support into that mix and it’s clear that the slew of endorsements for Romney in recent days have done something for the Massachusetts governor that winning primaries had not: Make clear he is the Republican presidential nominee.
Matt Lewis finds the Florida senator's endorsement premature:
In my estimation, this, perhaps ironically, makes Rubio a less appealing [VP] pick (should Romney be the nominee). Selecting a running mate can be a cathartic experience. But how can Romney now unite the GOP’s disparate factions by picking someone who’s already on his team?
Mataconis doubts that Rubio is secretly interested in the VP slot:
Indeed, considering that a losing Vice-Presidential candidate has only become President once in American history, one wonders why Rubio would take the offer at this point in his career. In any case, the party establishment is starting to rally behind Romney for real now. Next week, he has a chance to shut the lid on this race completely.