Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE (R-Ariz.) said Wednesday that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE played into the hands of Russian President Vladimir Putin by "attacking Montenegro & questioning our obligations under NATO."

McCain in a pair of tweets said that "the people of #Montenegro boldly withstood pressure from #Putin’s Russia to embrace democracy."

"The Senate voted 97-2 supporting its accession to #NATO," McCain said. "By attacking Montenegro & questioning our obligations under NATO, the President is playing right into Putin’s hands.

"Putin will do anything to shatter the transatlantic alliance," he added. "In 2016, he nearly succeeded in overthrowing #Montenegro’s democratically elected government & murdering its prime minister in order to prevent it from joining #NATO."

#Putin will do anything to shatter the transatlantic alliance. In 2016, he nearly succeeded in overthrowing #Montenegro’s democratically elected government & murdering its prime minister in order to prevent it from joining #NATO. Read more: https://t.co/0JO3bkFLI9 — John McCain (@SenJohnMcCain) July 18, 2018

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Trump during an interview that aired late Tuesday was asked by Fox News's Tucker Carlson why his son should "go to Montenegro to defend it from attack?"

"I've asked the same question," Trump responded. "Montenegro is a tiny country with very strong people. ... They are very aggressive people, they may get aggressive, and congratulations, you are in World War III."

Under Article 5 of NATO, every member nation must defend a fellow member nation if it is attacked. Article 5 has only been invoked after the 9/11 terrorist attacks in the U.S.

McCain in his tweet referred to a coup attempt in October 2016 that Montenegrin and U.S. officials have said was sponsored by Russia. NBC News reports that 14 people are currently on trial, accused of plotting to kill the prime minister and staging a coup in an effort to place a pro-Russia party in power.

The Montenegrin government has claimed that the coup was staged to stop it from joining NATO. The Kremlin has denied the allegations, according to NBC.

McCain has been an ardent defender of Montenegro's path to joining NATO. The country became the 29th member of the alliance in June 2017. In March, as McCain cited, the Senate approved Montenegro’s bid to join the alliance in a 97-2 vote.

But the vote did cause friction. Sen. Rand Paul (R-Ky.) had blocked a vote on the treaty, saying Montenegro would add to the U.S.'s military burden. McCain responded by saying his colleague “is now working for Vladimir Putin.”

McCain, who was diagnosed with brain cancer one year ago and has been away from the Capitol, has been highly critical of Trump for his meeting with Putin in Finland on Monday. The senator called Trump's press conference with the Russian president “one of the most disgraceful performances by an American president in memory.”