WASHINGTON — Two key Democratic lawmakers reintroduced legislation Thursday that would make it U.S. policy not to use nuclear weapons first.

Opponents, including top military leaders, argue the vague threat of nuclear escalation serves as a deterrent to conventional war as well as the use of chemical and biological weapons. But the sponsors ? House Armed Services Committee Chairman Adam Smith, D-Wash., and Senate Armed Services Committee member Elizabeth Warren, D-Mass. ? argue a “no first use” policy would lower the risk of an accidental nuclear war.

“The United States should never initiate a nuclear war,” Smith said in a statement. “This bill would strengthen deterrence while reducing the chance of nuclear use due to miscalculation or misunderstanding. Codifying that deterring nuclear use is the sole purpose of our nuclear arsenal strengthens U.S. national security and would renew U.S. leadership on nuclear nonproliferation and disbarment.”