U.S. Sens. Cory Gardner, R-Colo., and Richard Blumenthal, D-Conn., introduced a bipartisan resolution Wednesday, Feb. 8, pledging the United States’ unwavering commitment to the North Atlantic Treaty Organization (NATO). Sens. Claire McCaskill, D-Mo., and Marco Rubio, R-Fla., are original cosponsors of the resolution.

“It is indisputable that NATO is the most successful military alliance in history and it is essential that we strengthen NATO for current and future challenges to maintain global peace and stability. In order to achieve this goal, it is crucial that all NATO members make good on their commitment to contribute to NATO’s defense spending,” said Gardner in a news release. “Just last month, 4th Infantry Division troops based in Fort Carson were deployed to eastern Europe to enhance U.S. military presence and to offer support to our allies in the region. This resolution reaffirms our commitment to the NATO alliance as a force for good and to deter any and all aggression from adversaries.”

“Now more than ever, it is critical that the United States makes clear we stand with our NATO allies,” Blumenthal said in the release. “From building collective security during the Cold War to standing strong in the face of Russian aggression today, NATO has promoted peace, security and international cooperation for almost 70 years. This resolution will send a message to the world that our country will maintain strong leadership and strengthen its commitments to this historic organization.”

The resolution comes as White House national security adviser Michael Flynn is to recommend President Donald Trump support allowing the small Balkan nation of Montenegro to join NATO, according to POLITICO, and despite strong opposition from Russia. Trump, who criticized NATO as outdated during the presidential campaign, has praised Russian President Vladimir Putin and vowed to improve relations between Washington and Moscow. Trump recently agreed to attend a NATO summit in May.

The move will be a major test of the Trump administration’s policy toward Moscow, which considers any further eastward expansion of NATO a provocation. Trump could block Montenegro’s bid, since NATO’s rules require unanimous support from all members. If all NATO members approve Montenegro’s membership, Montenegro’s parliament must ratify the accord. A December 2016 opinion poll found only 39.5 percent of Montenegrins in favor of NATO membership and 39.7 against. Other opinion polls have suggested similar margins.

Most other NATO countries and the U.S. Senate widely support granting membership to the nation of 650,000 people, once part of the former Yugoslavia. Montenegro’s leaders have accused Putin of encouraging instability inside the country to erode support for joining NATO, including alleged plots by pro-Russian movements last year to attack the parliament and assassinate the prime minister.

U.S. Senators on both sides of the aisle see Montenegro’s bid as a test of resolve against increasingly belligerent Russian behavior, while U.S. intelligence agencies have accused Russia of trying to influence the recent U.S. presidential election by hacking and leaking the emails of Democratic officials and supporters of Hillary Clinton.