Michèle Flournoy is co-founder and managing partner of WestExec Advisors.  She served as the Under Secretary of Defense for Policy from February 2009 to February 2012.  Kingston Reif is the director for Disarmament and Threat Reduction Policy at the Arms Control Association.  The opinions in this commentary are their own.  View more opinion articles at CNN. 

(CNN) During his first two and a half years in office, President Donald Trump and his administration have laid waste to numerous international agreements originally designed to strengthen US security, bolster US alliances, and constrain US adversaries.  The toll has been particularly high with respect to deals concerning nuclear arms control and nonproliferation. 

Over the past 14 months, the administration has withdrawn from the 2015 Iran nuclear deal and abandoned the 1987 Intermediate Range Nuclear Forces (INF) Treaty .  Both of these valuable agreements have been discarded without a viable plan to replace them. 

Now the administration is signaling that it might jettison yet another nuclear pact, the 2010 New Strategic Arms Reduction Treaty (New START) with Russia.  Doing so would be the height of folly and would deal a significant blow to US national security.  With the US withdrawal from the INF Treaty having just taken effect on Aug. 2, New START will be the only remaining agreement constraining the size of the world's two largest nuclear arsenals.  Were New START to disappear, for the first time in nearly half a century there would be no legally binding limits on American or Russian nuclear stockpiles.  The risk of unconstrained US-Russian nuclear competition, and of even more tense bilateral relations, would grow. 

New START is one of the few remaining bright spots in the US-Russian relationship.  The treaty requires each side to reduce long-range nuclear forces to no more than 1,550 deployed warheads, 700 deployed long-range missiles and bombers, and 800 deployed and non-deployed missile launchers and bombers by Feb. 5, 2018—a deadline that both countries met

New START also includes a comprehensive monitoring and verification regime to ensure compliance.  But the agreement is set to expire on Feb. 5, 2021.  Under its terms, it can be extended by up to five years if both presidents agree. 

In an appearance before an activist group this summer, however, US National Security Advisor John Bolton, who before joining the administration called New START an "execrable deal," said that while no decision has been made, he thinks an extension is "unlikely." 

The decision to extend New START should be a no-brainer from both a security and budget perspective. 

The treaty caps the size of Russia's deployed nuclear arsenal and provides the US with information about Russia's forces that cannot be gained in any other way.  This reduces the Russian threat to the US and greatly aids American military and intelligence planning. 

For its part, Russia has repeatedly expressed interest in beginning talks on an extension of New START.  Unfortunately, Moscow has unhelpfully raised concerns about valid US procedures to remove some submarine-launched ballistic missile launchers and B-52 bombers from treaty accountability. 

But instead of taking Russia up on its offer to discuss prolonging the treaty, the White House is seeking a more comprehensive arms control deal that includes Russia and China and limits nuclear weapons not covered by New START, such as Russia's large arsenal of shorter-range "tactical" nuclear weapons. 

Pursuit of a broader agreement with Russia and China is worthwhile, but not at the expense of extending New START.  So far, the Trump administration does not appear to have a plan or the capacity to negotiate a new deal, which would likely take years.  Which raises the concern that the real goal of this gambit is to run out the clock on New START. 

Extending New START would buy the US five years of additional stability, predictability, and transparency.  It would also buy time to pursue a more ambitious agreement.  Throwing away the treaty's limits on Russian President Vladimir Putin's enormous existing arsenal of large, deployed nuclear weapons—just because it doesn't cover all Russian weapons or include China's much smaller arsenal —would be akin to cutting off our nose to spite our face. 

As the prospects for New START's renewal grow increasingly dim, Republicans and Democrats in Congress who support an extension should make it clear that ditching the treaty is unacceptable and pursue steps to guard against a new nuclear arms race. 

Congress should require reports from the intelligence community and the Pentagon on the implications of losing the agreement, as proposed in bipartisan legislation sponsored in the House by Foreign Affairs Committee Chairman Eliot Engel (D-N.Y.) and committee ranking member Rep. Mike McCaul (R-Texas) and in the Senate by Chris Van Hollen (D-MD) and Todd Young (R-Ind.).  Lawmakers should also make it clear that they won't provide funding to increase US nuclear forces above the treaty limits so long as Russia doesn't exceed them. 

Congress should also highlight a powerful economic argument for renewing New START. 

The Congressional Budget Office projects the cost of the Trump administration's nuclear sustainment and modernization plans to be $494 billion over the next decade—roughly $50 billion per year.  If New START is abandoned, and Russia decides to grow its nuclear arsenal, there will be pressure on the US to do the same.  This could prompt the already high costs to soar even higher, robbing resources from critical defense priorities, like ensuring that our military maintains its technological edge in a period of growing competition with Russia and China. 

New START is the last remaining piece of a US-Russian nuclear safety net painstakingly built and sustained by presidents from both parties over decades.  If this White House is intent on letting this safety net collapse, Congress must step in.  US national security demands no less.