Paul Ryan, like Rubio, endorses Mitt Romney because not doing so would improve Obama's re-election odds:
Weigel notes that this is not the norm:
Endorsements don't have to be rote. They're usually quite florid. The modern standard for a hell-yeah endorsement is Ted Kennedy's 2008 anointing of Barack Obama.
John Cassidy examines Romney's endorsements:
Romney’s campaign is understandably eager to show off all the people who are supporting him. But the actual endorsements bear inspection. Most of them are respectful. A few are warm and gracious. But many, particularly those from ardent conservatives, appear to have been drawn from the endorser with a pair of dental tweezers. Often, what they don’t say is as significant as what they do say.
He uses a rough taxonomy to classify Romney endorsers.