The libertarian and RINO wings of the Republican Party have a history of butting heads, but the ongoing feud between Kentucky senator Rand Paul and Arizona senator John McCain escalated to a new level on Thursday morning, when Paul appeared on MSNBC's "Morning Joe" show to defend himself against McCain's accusation that he was working for Vladimir Putin.

"I think he makes a really, really strong case for term limits," quipped Paul. "I think maybe he’s past his prime. I think maybe he’s gotten a little bit unhinged."

McCain accuses Paul of treason

The vitriol between the GOP senators has nothing to do with the Republican Obamacare replacement bill, the president's travel ban or allegations of Trump Tower wiretapping; at the center of the feud is the tiny nation of Montenegro.

On Wednesday night, McCain attempted to bring a treaty to the floor of the Senate over Montenegro's entry into NATO. McCain stated that anyone who opposes the measure is "carrying out the desires" of Vladimir Putin. Rand Paul blocked the unanimous consent motion that would have paved the way for Montenegro's NATO entry, thereby drawing the ire of the long-serving Arizona lawmaker. "I have no idea why anyone would object to this," scolded McCain from the Senate floor.

“The senator from Kentucky is now working for Vladimir Putin," announced McCain.

Rand Paul says the treaty goes against America's national interests

On his "Morning Joe" appearance on Thursday, Paul pointed out that the majority of Montenegro citizens were opposed to entering NATO and that the proposed treaty would "put a little angry McCain" all over the map.

Paul also reminded viewers that if Montenegro joined NATO, the United States would be required to come to the nation's defense if it should ever find itself embroiled in a war with Russia.

He then added that calling somebody a traitor for opposing the treaty would be viewed by most rational Americans as being "a little over the top".

According to Rand Paul, it's time for the 80-year-old McCain to retire.