It would also authorize the Senate Legal Counsel to challenge any attempt by the administration to withdraw from NATO without the Senate's consent in court.

"Just as it was required to join NATO, Senate approval should be required before this President – or any U.S. President - can withdraw,” Kaine said in a statement.

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McCain added that Trump's "mistreatment of our closest allies" had "raised doubts" about the U.S. commitment to NATO.

"In the future, the Senate must be prepared to defend its constitutional role. This legislation is urgently required to ensure that no president can withdraw the United States from NATO without the constitutionally required advice and consent of the Senate," he said.

Currently, presidents are required to get the consent of the Senate to enter into treaties. Article 13 of the NATO treaty requires a country give a one-year "notice of denunciation" before it can exit NATO.

Trump hasn't publicly threatened to withdraw from NATO but his rhetoric at a summit in Brussels earlier this month rattled long-time allies and stunned lawmakers.

And multiple reports indicated that he privately threatened to withdraw from NATO if other countries did not commit to increasing their defense spending. He did not deny those reporters during a press conference, instead saying he was "very firm" with allies.

“I think I can probably can [pull out of NATO], but that’s unnecessary, and the people have stepped up today like they’ve never stepped up before,” Trump added.

His rhetoric also sparked backlash on Capitol Hill, where the Senate used a non-binding procedural vote to signal their support for NATO.

Two House members introduced separate legislation on Thursday to prevent tax dollars from being used to withdraw the United States from NATO.