WASHINGTON — Two key Democratic lawmakers introduced legislation Wednesday that would ensure the U.S. does not fire nuclear weapons first in a potential future war.

House Armed Services Committee Chairman Adam Smith and Sen. Elizabeth Warren, a 2020 presidential hopeful and Senate Armed Services Committee member, offered a bill — “The No First Use Act” — to establish in law that it is the policy of the United States not to use nuclear weapons first in a conflict.

Though previous administrations have resisted such moves, and the GOP-controlled Senate is unlikely to take up the legislation, the players are notable. As chairman, Smith may elevate the issue by inserting the language into the annual defense policy bill, and Warren’s potential candidacy means the issue could reach the wider public on a future presidential debate stage.

Because the Trump administration’s Nuclear Posture Review states the U.S. reserves the right to use nuclear weapons in “significant non-nuclear strategic attacks,” such as attacks on the U.S., its allies and its nuclear infrastructure, some lawmakers have criticized that policy as over-broad.

“Our current nuclear strategy is not just outdated—it is dangerous,” Smith and Warren said in a joint statement. “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.”

Both lawmakers have previously advocated for military restraint and signaled in recent weeks that such a move was coming. Warren, called for a “no first use” nuclear weapons policy in a speech last month, and Smith did likewise in November, in a speech to the Ploughshares Fund, an anti-nuclear weapons group.

One of the arguments against President Barack Obama adopting such a policy when he considered in 2016 was that declaring it could undermine allies’ confidence in U.S. commitment to their defense—and spur them to pursue their own nuclear weapons. Removing the threat of nuclear escalation could embolden countries like North Korea, China, or Russia, who might believe that they could overwhelm U.S. allies before the U.S. could respond, the thinking goes.

U.S. Sen. Deb Fischer, R-Neb., a senior member of the Senate Armed Services Committee and the chairman of the Strategic Forces Subcommittee, blasted the proposed measure in a statement Wednesday. She pointed to presidents from both parties who have rejected it, “because it erodes deterrence, undermines allied confidence in U.S. security guarantees and risks emboldening potential adversaries.”

“Calculated ambiguity has long been an element of U.S. nuclear declaratory policy," Fischer said. “With Russia and China increasingly attempting to intimidate their neighbors – some of whom are U.S. allies – this is the wrong message to send. It betrays a naïve and disturbed world view.”

Fischer quoted the 2009 bipartisan Congressional Commission on the Strategic Posture of the United States, saying “Potential aggressors should have to worry about the possibility that the United States might respond by overwhelming means at a time and in a manner of its choosing.”

Should Smith seek to include the language in the 2020 National Defense Authorization Act, it could trigger pushback from other key Republicans, such as HASC’s ranking member, Rep. Mac Thornberry, and Senate Armed Services Committee chairman Sen. Jim Inhofe—who have both been supportive of the Trump administration’s direction on nuclear weapons.

The Union of Concerned Scientists hailed Smith and Warren’s bill in a statement on Wednesday. The organization said that such a policy would “reduce the risk of miscalculation during a crisis with Russia, China or North Korea; strengthen the Nuclear Nonproliferation Treaty by demonstrating the United States is serious about reducing the role of nuclear weapons in its security policy, and reduce risks associated with the president’s sole authority to order the use of nuclear weapons by removing the option of using them first.”

“The purpose of US nuclear weapons is to deter a nuclear attack on the United States and its allies. Any other purpose simply makes nuclear war more likely,” said Stephen Young, Washington representative for the UCS Global Security Program.