Story highlights Sen. Rand Paul on Monday accused fellow Sen. John McCain of McCarthyism.

McCain accused Paul of "working for (Russian President) Vladimir Putin" when Paul objected to a resolution on allowing Montenegro to join NATO.

(CNN) Sen. Rand Paul on Monday accused fellow Sen. John McCain of McCarthyism.

On the Senate floor earlier this month, McCain accused Paul of "working for (Russian President) Vladimir Putin" when Paul objected to a resolution on allowing Montenegro to join NATO. Paul later fired back at McCain by saying the 80-year-old senator made "a really strong case for term limits." He added he was "past his prime" and "gotten a little bit unhinged."

Paul did not back down when asked about McCain on the Sean Hannity radio program.

"You know, I thought the old McCarthyism got kind of old," Paul said. "Calling people communists, calling people friends of Russia, I thought that was so 1950s. I really thought we're beyond that. I take it pretty seriously because when somebody in the Senate calls another senator basically a traitor to their country, you think that somebody ought to be rebuked for that?"

A McCain spokeswoman didn't immediately respond to Paul's comment.