Sen. Elizabeth Warren, D-Mass., has sponsored a bill to prevent the U.S. from striking with nuclear weapons unless such a strike is used against it first.

WASHINGTON — Legislation introduced by Democratic lawmakers in the House and the Senate on Wednesday would bar the U.S. from using a nuclear weapon unless attacked with one first, demonstrating growing momentum for anti-nuclear sentiments on the left in the lead-up to the 2020 presidential election.

Sen. Elizabeth Warren, D-Mass., a 2020 presidential contender, and Rep. Adam Smith, D-Wash., the chairman of the House Armed Services Committee, introduced the “No First Use Act” in their respective chambers to codify in law what they said “most Americans already believe — that the United States should never initiate a nuclear war.”

The text of the bill is simple, saying only that “it is the policy of the United States not to use nuclear weapons first.” But while the measure has support among many Democrats, and could pass through the House, it likely wouldn’t win enough Republican backing to win approval in the Senate.

It almost certainly would face a veto by President Donald Trump, whose administration has opposed the proposal in its nuclear weapons policy. Still, the introduction of the bill by a high-profile presidential contender and the chairman of one of the House’s most powerful committees indicates how the idea is gaining traction within a swath of the Democratic base.

“Our current nuclear strategy is not just outdated — it is dangerous,” Warren and Smith said in a joint statement regarding the bill. “By making clear that deterrence is the sole purpose of our arsenal, this bill would reduce the chances of a nuclear miscalculation and help us maintain our moral and diplomatic leadership in the world.”

President Barack Obama considered declaring a “no first use” policy for nuclear weapons in the final year of his administration. Members of his administration, including Secretary of State John Kerry and Defense Secretary Ash Carter, and U.S. allies opposed the measure. Obama ultimately decided against pursuing it.

The election of Trump that same year raised concerns about the president’s authority to launch a nuclear weapon, a reality that Democratic candidate Hillary Clinton made a centerpiece of her 2016 campaign’s messaging, warning that Trump didn’t have the temperament to have his finger on the nuclear button.

Democratic lawmakers have seized upon such concerns since Trump’s election.

Sen. Edward Markey, D-Mass., and Rep. Ted Lieu, D-Calif., this week reintroduced a bill they had put forth before that would prevent the president from launching a nuclear first strike without first receiving congressional approval. Theoretically, the president must receive approval from Congress to declare war, but past presidents have found legal grounds to take military action without that signoff.

China and India have adopted “no first use” policies, but questions have arisen of late about whether their governments will retain them. The U.S., Russia, France, Pakistan and Britain reserve the right to conduct a first strike in extreme circumstances.

Those who want to retain Washington’s right to strike first with a nuclear weapon in a conflict say the strategic ambiguity helps bolster deterrence and argue that there may be circumstances in which a nonnuclear strike on strategic assets or population centers warrants a nuclear response.

The Trump administration determined that a “no first use” policy wasn’t warranted when it released its nuclear weapons policy in early 2018. The policy said the U.S. only would consider employment of nuclear weapons in extreme circumstances, which could include “significant nonnuclear strategic attacks,” such as attacks on infrastructure, nuclear forces, their command and control or warning capabilities.

“To help preserve deterrence and the assurance of allies and partners, the United States has never adopted a ‘no first use’ policy and, given the contemporary threat environment, such a policy is not justified today,” the Trump administration’s Nuclear Posture Review concluded. “It remains the policy of the United States to retain some ambiguity regarding the precise circumstances that might lead to a U.S. nuclear response.”

Supporters of a “no first use” policy in the U.S., however, argue that it would reduce the risk of miscalculation by an adversary during a crisis and would strengthen deterrence by making clear under what circumstances Washington would resort to nuclear war.

“There’s a growing danger that in a crisis, American or Russian leaders will use nuclear weapons first out of concern that the other guy is going to go first,” said Jon Wolfsthal, senior adviser to the anti-nuclear group Global Zero and a former senior director for arms control and nonproliferation on Obama’s National Security Council. “There are lots of ways you can reduce that fear. ‘No first use’ is a very low-cost way of raising the nuclear threshold and reducing the chances that a crisis goes nuclear.”