WASHINGTON — As U.S. President Donald Trump has sent letters to Western leaders warning them to increase defense spending, ratcheting up tensions for a NATO alliance in Brussels this week, U.S. lawmakers are urging him to back the alliance.

Looming large is Trump’s other summit, with Russian President Vladimir Putin in Helsinki, set for after the Brussels meeting.

Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., said in a tweet on Monday that “the U.S. must stand firmly with our NATO allies and affirm our transatlantic partnership. Doing otherwise strengthens Putin and undermines democratic values.”

For his part, NATO Secretary General Jens Stoltenberg told CNBC he’s not expecting Trump to go on the attack about burden-sharing, as Trump recognizes the issue is moving in the right direction.

“The good news is that we have started to do exactly that and we have started to see a significant increase,” Stoltenberg said. ”Over the past few years we have added $87 billion from Canada and the European allies, and that makes a real difference.”

In any case, the Senate Foreign Relations Committee next week is set to take up a nonbinding sense-of-the-Senate resolution emphasizing the strategic importance of NATO to the collective security of the transatlantic region — and urging its member states to cooperate to strengthen the alliance.

A group of senior Senate Democrats accused Trump in a July 2 letter of sending mixed messages ahead of the two summits, though its unlikely Trump will take heed. They are Sens. Bob Menendez, D-N.J., the ranking member of the Senate Foreign Relations Committee; Dick Durbin, D-Ill., the Senate Democratic whip; Mark Warner, D-Va., the vice chairman of the Senate Select Committee on Intelligence; and Jack Reed, D-R.I., the ranking member of the Senate Armed Services Committee.

“Clear U.S. leadership is necessary to counter President Putin’s efforts to undermine the NATO alliance and its founding values,” they wrote. “It is imperative that you make a strong statement of support for the democratic nations that make up the alliance and make clear that the United States stands with — not in opposition to — our oldest and closest allies.”

The House Armed Services Committee’s ranking member, Rep. Adam Smith, D-Wash., said on CNN last week that he was troubled Trump appears to be ”trying to reorient U.S. foreign policy towards Russia and away from our democratic allies in Europe.”

“I believe the president is lining us up with an authoritarian dictator instead of with democracies that promote economic and political freedom,” Smith said. “I don’t think that’s what the United States should stand for. And I don’t think it’s in our long-term best interests in terms of our policy.”

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

More In NATO Priorities