John McCain. Win McNamee/Getty Images Republican Sen. John McCain slammed his GOP colleague, Kentucky Sen. Rand Paul, on Wednesday after Paul blocked the passage of a treaty that would allow Montenegro to move forward with joining NATO, Defense News' Joe Gould reported.

McCain warned before the vote that “If there’s objection, you are achieving the objectives of Vladimir Putin...and I do not say that lightly.”

Paul then entered the Senate chamber, voted against the accession protocol, and exited.

“The only conclusion you can draw when he walks away is he has no argument to be made,” McCain said after Paul walked out abruptly, according to The Washington Examiner. "The senator from Kentucky is now working for Vladimir Putin."

Russian President Vladimir Putin is staunchly opposed to Montengro's accession to NATO, which he views as a threat to Russian sovereignty. Albania and Croatia joined the alliance 2009.

"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," McCain's spokesperson, Julie Tarallo, told Business Insider on Wednesday.

"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," Tarallo added.

For Montenegro to move forward with the accession process, which was approved in May, the treaty has to be ratified by the US Senate by unanimous consent. Twenty-one of 28 NATO allies have already backed Montenegro's accession.

Asked about McCain's comment, Paul stood by his decision to block the treaty and said it would be "unwise to expand the monetary and military obligations of the United States given the burden of our $20 trillion debt."

"Currently, the United States has troops in dozens of countries and is actively fighting in Iraq, Syria, Libya, and Yemen (with the occasional drone strike in Pakistan)," Paul told Business Insider in a statement provided by his office. "In addition, the United States is pledged to defend 28 countries in NATO."

McCain promised Montenegro Prime Minister Dusko Markovic in January that the Senate would ratify the treaty soon, according to Radio Free Europe.

Markovic was the target of a coup plot, allegedly orchestrated by Russian intelligence agents, to overthrow Montenegro's pro-NATO government and replace it with a Russia-friendly regime, The New York Times reported. The plot was uncovered in November after the pro-Russian mercenary who helped organize it was arrested by Montenegrin police.