President Trump and Russian President Vladimir Putin arrive for a meeting in Helsinki on July 16. (Brendan Smialowski/AFP/Getty Images)

A quartet of senators launched a new bipartisan effort Thursday to prevent President Trump from withdrawing the United States from NATO without the prior approval of the Senate, the latest effort to constrain the president from upending U.S. policy regarding Russia.

The bill would require the president to secure the support of two-thirds of the Senate — the same threshold required to enter into a treaty — before he could withdraw from the nearly 70-year-old alliance. It also authorizes the Senate’s legal counsel to represent the body in any court cases needed to prevent a withdrawal from NATO without the Senate’s approval.

The measure was drafted by Sens. Cory Gardner (R-Colo.) and Tim Kaine (D-Va.), both of whom sit on the Senate Foreign Relations Committee; Senate Armed Services Committee Chairman John McCain (R-Ariz.) and ranking Democrat Jack Reed (R.I.) have also signed on to the measure as leading co-sponsors.

“Regrettably, President Trump’s mistreatment of our closest allies has raised doubts about America’s commitment to the transatlantic alliance and the values of defense,” McCain said in a statement. McCain, despite presently undergoing treatment for brain cancer, remains Congress’s most respected statesman and has been a frequent critic of the president’s stances regarding NATO and Russia.

“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 continued.

While Senate consent is required to enter into treaties, the Constitution is silent on the question of how the United States is supposed to exit a treaty. Article 13 of the NATO treaty does address the question of when countries may exit — any country may leave the treaty one year after giving a “notice of denunciation” to the United States, which is then supposed to inform every other member — but it does not specify what national authority has the power to issue that notice of denunciation.

Trump has not publicly threatened to pull the U.S. from NATO, and at this month’s summit in Brussels, he signed on to various efforts to counter the Kremlin and rededicate the U.S. to the organization’s collective defense. But he has also repeatedly voiced doubts about the strategic importance of the organization, haranguing allies for not dedicating more funding ahead of schedule and even questioning its continued relevance, as he makes un­or­tho­dox overtures to improve relations with Russian President Vladi­mir Putin.

Last week, Trump even said on Twitter that a summit with Putin in Helsinki “may prove to be, in the long run, an even greater success” than his just-completed meetings with NATO allies.

The combination has proved troubling to more senators than just those who signed on to to the NATO bill unveiled Thursday. Earlier this week, Sens. Robert J. Menendez (N.J.), the top Democrat on the Foreign Relations Committee, and Lindsey O. Graham (R-S.C.) announced that they were working on a measure that would also require Senate approval for any bid to withdraw the U.S. from NATO, while also stepping up sanctions on Russian oligarchs and the country’s energy, financial and cyber sectors.

The NATO-focused measures are only a subset of the bipartisan proposals lawmakers have unveiled in the wake of Trump’s Helsinki summit with Putin, during which he suggested that he might take the word of the Russian leader over the conclusions of his own intelligence community regarding Russian election interference and influence campaigns. Lawmakers are still trying to get the administration to tell them what transpired during the two-hour, one-on-one meeting that preceded that surprise announcement, and an ensuing series of attempted walk-backs as the administration tried to address Republican criticism labeling Trump’s performance as everything from “shameful” to a “missed opportunity.”

On Wednesday, members of the Senate Foreign Relations Committee grilled Secretary of State Mike Pompeo for details about the meeting, which he said he had been briefed on, and for an explanation as to why the White House appeared to be “making it up as they go” when it came to foreign policy, particularly concerning Russia.

“The administration tells us, ‘Don’t worry, be patient, there is a strategy here.’ But from where we sit, it appears that in a ‘ready, fire, aim’ fashion, the White House is waking up every morning and making it up as they go,” Sen. Bob Corker (R-Tenn.), the panel’s chairman, told Pompeo, complaining that the president’s actions “create tremendous distrust in our nation, among our allies, it’s palpable.”

“Is there a strategy to this?” Corker asked.

In the meantime, the House voted overwhelmingly Thursday to approve an annual defense authorization bill that reaffirmed the United States’ commitment to NATO and to increasing the defensive capabilities of European allies. The Senate is expected to take up the measure late next week and send it to the president’s desk.