The latest wave of opposition to the lower-yield W76-2 warhead the Navy deployed last year crested Wednesday, when five Senators asked Secretary of Defense Mark Esper what the Pentagon might use the weapon for, and which civilian nuclear work had to be postponed to build it.

The five senators, four Democrats and an independent who caucuses with Demcorats, posted their letter to Esper online Wednesday afternoon

Sen. Dianne Feinstein (D-Calif.), lead signee on the letter, was joined by: Sen. Patrick Leahy (D-Vt.), chair of the Appropriations Committee; Sen. Bernie Sanders (I-Vt.), the progressive firebrand and leading Democratic presidential hopeful; Sen. Tammy Duckworth (D-Ill.), the service-disabled veteran who sits on the Armed Services Committee; and Sen. Elizabeth Warren (D-Mass.), another Armed Services member and contender for the Democratic presidential nomination.

Aside from probing Esper about the W76-2’s effect on the NNSA pipeline — the civilian agency pushed the 76-2 mod through while the assembly line at Pantex was still rolling from the W76-1 life-extension — the Senators wanted to know whether a Trident missile armed with a W76-2 would, despite its lower yield, count as a strategic weapon if deployed on a strategic delivery vehicle.

The Pentagon, in the 2018 Nuclear Posture Review, said it needs a lower yield nuclear weapon that can rapidly penetrate enemy air defenses to deter Russia from quickly escalating and winning a conventional war with its own lower yield nuclear strike.

Like the W76-1, W76-2 is used on Trident II D5 missiles carried aboard Ohio-class ballistic missile submarines, which are covered by the New START nuclear-arms control treaty between Russia and the U.S., which will expire in less than a year unless President Trump and Vladimir Putin, the president of Russia, agree to extend it.

The Trump administration continues to push for a new treaty that includes China, which has repeatedly said it wants no part of such a treaty, and exotic Russian weapons such as nuclear-powered, nuclear-armed autonomous undersea vehicles, that are not covered by New START.

The treaty limits Moscow and Washington to no more than 1,550 warheads across 700 intercontinental ballistic missiles, submarine-launched ballistic missiles, and heavy bombers.

The Pentagon confirmed this month that it deployed the w76-2 in December 2019. The nonprofit Federation of American Scientists estimates the weapon has a yield of 6 kilotons or so, compared with around 100 kilotons for the W76-1.

The NNSA spent roughly $75 million on W76-2 in fiscal years 2019 and 2020 and plans to wrap up work associated with the lower-yield weapon before the current fiscal year ends Sept. 30.