Lieberman said he sees Rubio rapidly becoming part of a new “deep bench” of foreign policy experts in the Republican caucus, especially after it lost Indiana Sen. Dick Lugar, who was just ousted in a primary tea-party rout. Lieberman said Rubio is “unique.”

But, in some ways, Rubio’s retracing the steps of Hillary Clinton, who arrived to the Senate a rookie but as a rock-star politician. She’s now the ultimate foreign-policy official: secretary of state. Like Clinton, Rubio blended into the Senate for the first few months in office, shied away from the spotlight and immersed himself in his work.

Just after the Brookings speech, Rubio joined with Democratic Sen. Bob Casey of Pennsylvania to author a resolution condemning the atrocities in Syria. Rubio, Lieberman said, helped overcome Republican objections. Case said in a written statement that he was happy with the “bipartisan effort” against Syria and Iran.

“While we disagree on many issues,” Casey, a fellow foreign relations committee member, said, “he has welcomed the opportunity to work together in a bipartisan way on these critical national security issues.”

As evidenced by the written statement, Casey was far more hesitant to discuss his Republican colleague in an interview. Other Democratic senators weren’t willing to comment at all about the man who could be the number two on the opposition ticket.

Still, Rubio has a strong working relationship with fellow Florida Sen. and intelligence committee member Bill Nelson, who faces a tough reelection. And Kerry and Lieberman said Rubio seems ready to be on this year’s ballot as well.

"If you’re up to it, I don’t think there’s pressure,” Kerry said. “I think he’s handling it well. I don’t see any sign that it’s pressure.”