Senate Majority Leader Mitch McConnell, R-Ky., invoked an obscure rule last month barring lawmakers from slandering each other on the floor of the U.S. Senate.

He reminded Sen. Elizabeth Warren, D-Mass., of Rule XIX, and then enforced it, after she tried repeatedly to read a letter criticizing former Sen. Jeff Sessions, R-Ala.

In contrast, it appears that Sen. John McCain, R-Ariz., will avoid a similar rebuke from leadership for claiming this week that Sen. Rand Paul, R-Ky., works for Russian President Vladimir Putin.

The junior senator from Kentucky objected to a bill Wednesday that would advance Montenegro's pursuit of NATO membership. After Paul objected, he exited the chamber immediately, which drew sharp criticism from McCain.

"I note the senator from Kentucky leaving the floor without justification or any rationale for the action he has just taken. That is really remarkable, that a senator blocking a treaty that is supported by the overwhelming number — perhaps 98, at least, of his colleagues — would come to the floor and object and walk away," McCain said.

The Arizona senator added, "The only conclusion you can draw when he walks away is he has no justification for his objection to having a small nation be part of NATO that is under assault from the Russians. So I repeat again, the senator from Kentucky is now working for Vladimir Putin."

Paul fired back Thursday morning during an interview on MSNBC, saying, "You know, I think [McCain] makes a really, really strong case … for term limits. I think maybe he's past his prime. I think maybe he's gotten a little unhinged."

McCain's remarks were ugly. Paul's response was ugly.

However, of the two, the Arizona senator's comments deserve a response from leadership, as they were spoken from the floor of the U.S. Senate.

If leadership found it worth rebuking Warren for reading a letter, then surely they see a similar problem with McCain accusing a colleague of working to benefit a hostile foreign agent.

The specific rule that was cited last month against Warren states that, "No Senator in debate shall, directly or indirectly, by any form of words impute to another Senator or to other Senators any conduct or motive unworthy or unbecoming a Senator."

McCain's attack on Paul appears to be a clear violation of this rule.

It's worth noting that McConnell invoked Rule XIX against Warren only after she was given multiple warnings. McCain, on the other hand, leveled his attack on Paul and then yielded his remaining time.

Still, it's a question of the spirit of the law. If the argument for rebuking Warren was that decorum and senatorial comity must be recognized and protected, then there's no reason why McCain's attack on Paul should go unanswered by leadership.

The Arizona senator's office is not backing down, and they continued to defend his remarks Thursday.

"Senator McCain believes that the person who benefits the most from Congress's failure to ratify Montenegro's ascension to NATO is Vladimir Putin, whose government has sought to destroy the NATO alliance, erode confidence in America's commitments to its allies, overthrow the duly-elected government of Montenegro, and undermine democratic institutions throughout Europe," a spokeswoman for the senator, Julie Tarallo, told the Washington Examiner.

She added, "The overwhelming majority of senators who support this treaty, and certainly the people of Montenegro, deserved an explanation from Senator Paul on the Senate floor as to why he sought to prevent this small, brave country from joining in the defense of the free world."

Paul's office declined to comment. Spokespersons for McConnell did not respond to the Examiner's request for comment.