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: