Democratic presidential hopeful Senator Elizabeth Warren proposed legislation Wednesday that would bar the US government from using nuclear weapons first in a conflict.
"Our current nuclear strategy is not just outdated -- it is dangerous, Warren and Smith said in a statement.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."
In the divided Congress, passing the law, which would be seen by conservatives as tying the US military's hands on battle strategy, would be difficult.
And President Trump would likely veto it.
Trump has showed a determination to maintain US military advantage over its rivals, including in the nuclear sphere.
Last year, he announced he intended to pull out of the 1987 Intermediate-Range Nuclear Forces treaty with Russia, accusing Moscow of violating the pact to gain advantage.
Warren's bill gives the 69-year-old Massachusetts senator a chance to lay out her defense stance as she prepares for an expected run for the White House in the 2020 election, positioning herself as a progressive.
The proposal came as the Trump administration wrestles with an increasingly complex nuclear rivalry with Russia and China.
In non-proliferation talks in China earlier Wednesday, Washington accused Moscow and Beijing of not fully reporting their nuclear programs and called for more transparency, in order to enforce the 1970 Non-Proliferation of Nuclear Weapons treaty.
Separately, on Tuesday Senator Ed Markey and Congressman Ted Lieu, both Democrats, introduced legislation that would prevent the US president from launching a nuclear first strike without Congress's approval.
Currently, the decision to unleash a nuclear weapon is the president's alone.
"Trump's brand is to be unpredictable and rash, which is exactly what you don't want the person who possesses the nuclear football to be," said Lieu, referring to the nickname for the briefcase always kept close to the president that carries the mechanism and codes needed to launch a nuclear strike.
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