The amendment, introduced by Sen. Rand Paul (R-KY), states: "It is the sense of Congress that Article 5 of the North Atlantic Treaty does not supersede the constitutional requirement that Congress declare war before the United States engages in war."

The amendment failed in a vote of 16-83 and received no support from Democrats.

The Senate also approved an amendment introduced by Sen. Tim Kaine (D-VA) that requires Senate approval for a president to leave NATO in a vote of 65-28, with only Republicans opposing the measure.

The Kaine amendment reads: "The President shall not suspend, terminate, denounce, or withdraw the United States from the North Atlantic Treaty… except by and with the advice and consent of the Senate, provided that two-thirds of the Senators present concur, or pursuant to an Act of Congress."

Versions of the Kaine and Paul amendments were previously introduced as stand-alone bills. When Paul and several other Republicans introduced legislation to reaffirm congressional war powers over NATO commitments, he noted that while NATO members are required to assist each other in the event of an attack, military action is not mandated.

"Furthermore, Article 11 of the NATO Treaty states that the provisions of the Treaty are to be carried out in accordance with each country's respective constitutional processes," Paul said in a statement last month.