Twenty-four Democratic senators urged President Trump on Friday to renew a nuclear arms treaty with Russia that is set to expire in two years.

“We write to urge your administration to extend the New Strategic Arms Reduction Treaty (New START) with Russia for another five years in order to ensure that the United States can continue to enjoy the treaty’s clear national security benefits over this period,” the senators wrote in a letter to Trump.

“We are very concerned that so far, the United States has made no visible or concrete effort to extend its life” they added. “Failure to extend New START risks unraveling a broader arms control regime that has helped uphold stable deterrence and curb a costly, destabilizing arms race for nearly half a century.”

The letter was organized by Sen. Chris Van Hollen (D-Md.). Co-signers include several senators running for the Democratic Party’s 2020 presidential nomination — Sens. Kirsten Gillibrand (D-N.Y.), Amy Klobuchar(D-Minn.), Bernie Sanders (I-Vt.), Elizabeth Warren (D-Mass.), Cory Booker(D-N.J.) and Kamala Harris (D-Calif.).

The Obama-era New START caps the number of nuclear warheads the United States and Russia can deploy at 1,550 each, among other provisions.

The treaty expires in 2021, but there is an option to extend it another five years after that.

Arms control advocates are worried Trump will let New START expire after he moved to withdraw from a separate arms control treaty with Russia earlier this year.

If both New START and the other treaty, known as the Intermediate-range Nuclear Forces (INF) Treaty, end, it would be the first time in decades the world’s largest nuclear powers have no treaty limitations on their arsenals.

In the case of the INF Treaty, Russia has been accused of violating the deal for years. But there have been no similar accusations related to Russia’s compliance with New START.

Indeed, this week, Secretary of State Mike Pompeo told senators that Russia is largely complying with New START.

But he also indicated to the Senate Foreign Relations Committee that the Trump administration wants to expand the scope of New START in order to renew it. Specifically, he cited new technologies being developed by Russia, as well as wanting to fold in other countries, particularly China.

Also, in 2017, Trump called New START a “one-sided deal” that was “just another bad deal” made by former President Obama.

In their letter, the senators argued New START has “advanced the security interests of the United States and underpinned strategic stability” with Russia without “inhibiting the ability of the United States to maintain a survivable, reliable, and effective nuclear deterrent.”

The senators also highlighted repeated congressional testimony from Strategic Command chief Gen. John Hyten on the benefits of New START, such as the verification regime giving the United States insight into Russian capabilities.

They also argued extending the treaty for five years will give the United States room to negotiate a follow-up deal to address the issue of new technologies.

“Arms control is not an end in itself; it is a tool for containing the military capabilities of our adversaries and safeguarding the national security interests of the United States and its allies,” they wrote. “Since 1972, Republican and Democratic administrations alike have pursued such measures as a complement to maintaining a robust nuclear deterrent. We urge you to sustain this bipartisan policy and advance U.S. security by extending New START for an additional five years.”