Cracks in Rubio’s hawkish foundation started to show during the debate earlier this year over foreign aid to Egypt. Paul has fervently opposed sending aid, while Rubio has sought a middle ground in which the aid would continue under more limited circumstances because the U.S. can’t “retreat” from its role on the world stage.

But with the stakes in Syria much higher, Rubio is struggling to juxtapose his proclivity toward foreign intervention with criticism of the president’s handling of the crisis. If the timing is good for Paul and Texas Sen. Ted Cruz, another potential presidential candidate opposed to military action in Syria, it couldn’t be worse for Rubio, who is weathering a backlash from his own party for backing immigration reforms also favored by President Obama. Rubio has balanced that position out with a strident demand that Republicans who oppose the president’s health care law should reject the federal budget.

“Any accommodation, any agreement with the president is a liability in a Republican primary,” said Republican consultant Steve Schmidt, a top adviser to McCain’s 2008 presidential campaign. “The potential candidates in 2016 like Rubio and Paul will be taking positions on Syria and other issues that will come to define them and expose real fault lines long before the primary debates.”