Sen. Rand Paul called fellow Republican Sen. John McCain "a little bit unhinged" Thursday after McCain had said "the senator from Kentucky is now working for Vladimir Putin," the Russian president.
He also said McCain, who is 80, may be "past his prime."
The discord between two senators from the same party is remarkable even in the current political environment.
Paul's comments came in an interview on MSNBC's "Morning Joe." He was responding to remarks McCain made a day earlier while seeking a Senate vote on including Montenegro in NATO.
Russia opposes Montenegro's inclusion in NATO. As debate ensued, McCain saw Paul come to the Senate floor and remarked that anyone who objected to his request would be carrying out "the desires and ambitions of Vladimir Putin, and I do not say that lightly."
Paul objected nonetheless, a parliamentary obstacle to McCain's request for a future vote.
The senator from Kentucky did not explain his actions and left the Senate chamber, which only seemed to increase McCain's disdain. He said Paul was blocking a treaty supported by an overwhelming number of senators and repeated the assertion about working for Putin. Two Democratic senators -- Jeanne Shaheen of New Hampshire and Ben Cardin of Maryland -- also criticized Paul.
"I have to agree with Senator McCain. He is working in support of Russia's interest in America or he is holding this hostage for something that is totally unrelated to what we are doing with Montenegro's accession into NATO. In either case, it is totally inappropriate," Shaheen said.
Paul said in Thursday's interview that McCain in his remarks has made "a really, really strong case for term limits. I think maybe he's past his prime. I think maybe he's gotten a little bit unhinged." McCain was elected to a sixth term last year.
Paul went on to argue that McCain's foreign policy views "greatly overextend us." Paul questioned whether it would be in the best interest of the United States to get involved in a war if Montenegro were involved in an altercation with another country.
It's not the first time the pair had a war of words. McCain once said it's always the "wacko birds" on the right and left who get the media megaphone, and then made clear he was talking about Paul, Sen. Ted Cruz, R-Texas, and Rep. Justin Amash, R-Mich.