Sen. Rand Paul (R-Ky.), who is campaigning with Mitt Romney this week, wrote an op-ed for making clear that he doesn't support the Republican presidential nominee on every issue. 

Responding to Romney's recent speech on foreign policy, Paul wrote, "Romney chose to criticize President Obama for seeking to cut a bloated Defense Department and for not being bellicose enough in the Middle East, two assertions with which I cannot agree."

The senator argues against intervention in Syria; Romney argued for arming rebels. 

Paul endorsed Romney in June -- before his father, Rep. Ron Paul (R-Tex.), had ended his own presidential bid. The elder Paul's campaign manager just went to work for Sen. Mitch McConnell (R-Ky.), a sign that the libertarian-leaning Pauls are moving closer to the establishment. But with this op-ed, Rand Paul -- seen by many as the future leader of his father's grass-roots movement -- makes it clear that he won't always toe the party line. 

Within the GOP, Paul's foreign policy advocacy is already ruffling feathers. Paul's political action committee cut an ad attacking Sen. Joe Manchin (D-W.Va.) for voting against legislation that would cut aid to Egypt, Pakistan and Libya. Sen. Lindsay Graham (R-S.C.) crossed the aisle to publicly defend Manchin, saying the bill in question (which many Republicans opposed) would endanger the peace treaty between Egypt and Israel.