Newt Gingrich said Rep. Paul Ryan's endorsement of rival Mitt Romney is "a great achievement" for the former Massachusetts governor.

"Paul Ryan is one of the rising stars in the Republican Party," Gingrich said during an interview on Charlie Sykes's radio show Friday. Ryan's endorsement came just days ahead of the GOP primary in Ryan's home state of Wisconsin.

Gingrich also noted the importance of Sen. Marco Rubio's endorsement of Romney, which came earlier this week. After backing Romney, Rubio (R-Fla.) hinted that he might have preferred someone else as the GOP nominee, saying "there are a lot of other people out there that some of us wish had run for president, but they didn't."

The former House Speaker admitted Romney "is clearly the frontrunner" but still expressed doubts about his rival's ability to earn all the delegates needed to nab the Republican nomination.

"I have no doubt that once he gets 1,144 delegates we will all unify and support him," Gingrich said. "I think Mitt Romney is clearly the frontrunner. I think he will probably get 1,144 but I think he has to earn it."

(h/t The Hill)

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