A bill that would restrict U.S. presidents’ sweeping authority to use nuclear weapons has been introduced by two Democrats in Congress.

The bill would restrict a president’s ‘first strike’ authority, or the ability to use nuclear weapons before an enemy launches a nuclear attack on the United States, to situations where Congress has already declared war.

“It is a frightening reality that the U.S. now has a Commander-in-Chief who has demonstrated ignorance of the nuclear triad, stated his desire to be ‘unpredictable’ with nuclear weapons, and as President-elect was making sweeping statements about U.S. nuclear policy over Twitter,” California congressman Ted Lieu said in a statement.

As things stand, presidents have unrestricted authority to use nuclear weapons as they see fit.

“They are concerned about Trump going off the deep end and ordering a first use of nuclear weapons in situations in which the United States is not under attack,” explains Bruce Blair, a Princeton University professor who is a former nuclear launch officer. Tweet This

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“His temperament, and compulsiveness, defensiveness and lack of knowledge about world affairs and nuclear weapons might lead him to order the use of nuclear weapons in a wholly inappropriate circumstance, and there’s nothing to stop the president from issuing such a directive.”

Trump’s comments about the U.S. nuclear arsenal, and how he sees his authority over it, have been confusing.

He has said repeatedly that a president’s use of nuclear weapons should be “unpredictable.” If nuclear weapons can’t be used, “why are we making them?” he asked on another occasion.

In September, he appeared to renounce first use, but then said that he “can’t take anything off the table.”

And in December, he called for an international nuclear arms race, saying that the United States “will outmatch them at every pass and outlast them all.”

The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes — Donald J. Trump (@realDonaldTrump) December 22, 2016

With Republicans controlling both houses of Congress and the White House, the bill is unlikely to make it into law, Blair cautions.

“It’s hard to believe that a Republican-controlled Congress would pass legislation like this. It would be vetoed, probably, by the president, and would probably be the end of it.”

READ: Donald Trump’s call for nuclear ‘arms race’ perplexes nuclear experts

A U.S. president has sweeping power to decide whether, when and how to use nuclear weapons. (While the U.S. system of government otherwise is based on checks and balances, the president’s most extreme power is also his least constrained.)

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Before his inauguration, Trump was briefed on his nuclear authority and the details of how he would order a launch. He is followed at all times by a military officer carrying the ‘football,’ a communication device that he could use to order nuclear weapons into the skies.

U.S. President Donald Trump leaves CIA headquarters on January 21, 2017 in Langley, Virginia. In the background a military officer carries the ‘football’, with launch codes for nuclear weapons. GETTY IMAGES

Under the harsh Cold War logic that governs nuclear decision-making, an enemy would have to fear that the U.S. could retaliate even if it was targeted by a devastating nuclear attack. So the president is given authority to launch American missiles aimed at an enemy while the enemy’s missiles are still in the air. A president might have less than 10 minutes to make a world-changing decision, perhaps after being woken from a deep sleep.

However, the system assumes perfect decision-making, based on perfect information. Both have failed.

Presidents, being human, have fallen short of perfection. Richard Nixon succumbed to dark moods and heavy drinking as his presidency neared its humiliating end. Then-defense secretary James Schlesinger quietly ordered the Pentagon not to act on a nuclear launch order from the White House unless officers there had talked to him first. Ronald Reagan may have begun to suffer from Alzheimer’s disease while president, though if there were constraints placed on his nuclear authority they have never been discussed in public.

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And nuclear alerts have been triggered by accident. In 1980, a defective computer chip persuaded the Pentagon that over 2,200 Russian missiles were on the way. Another was caused in 1979 when a computer tape simulating a nuclear attack was mistaken for the real thing.

It also assumes both sides have a certain level of rational predictability, says Royal Military College professor Christian Leuprecht.

“Mutual assured destruction works when you can count on reasonably rational behaviour by all the actors. In the context of the uncertainty that Trump has introduced, that would introduce uncertainty into how he might respond to a genuine threat.”

WATCH: Donald Trump only added to fears of a new nuclear arms race when asked to clarify a tweet by an MSNBC anchor. Vladimir Putin dismissed Trump’s bluster as nothing new, but added Russia can obliterate U.S. missile defence systems. As Shirlee Engel reports, the Cold War rhetoric is raising questions about just how warm relations will remain between the two leaders.

2:15 Donald Trump doubles down on calls to increase nuclear arms Donald Trump doubles down on calls to increase nuclear arms

Leuprecht raises another possibility — a nuclear launch that’s a mistaken response to a conventional attack.

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“It’s entirely possible that if the Russians were really pissed about something that the Americans did, or that the West did, that they would launch a conventional missile into some military installation just to say ‘If you don’t stop, who knows what is going to be next?’ The problem, of course, is that on the U.S. end, and the NORAD end, there is no way to know whether these are nuclear-armed or not.”

Trump’s comments about nuclear weapons, and his willingness to lash out at provocations, have caused concern about his total power over the U.S. nuclear arsenal of over 4,000 warheads.

“A man you can bait with a tweet is not a man we can trust with nuclear weapons,” Democratic rival Hillary Clinton said during the campaign.

READ: Why a ‘nuclear mutiny’ against a President Donald Trump would likely fail

Blair is unsettled by the combination of Trump’s personality and his nuclear authority.

“It’s his temperament, his nuclear authority, his sort of mindset, if that’s the right word, which divides the world into winners and losers. You see this kind of attitude in the nuclear arena expressed when he said that the United States would out-compete any other country in a nuclear arms race. He seemed quite open to the possibility that it wouldn’t be a very big deal if we embarked on a nuclear arms race.”

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“He thinks that we would win it, and I think he believes that he’s such a superior negotiator, a deal maker, that he believes he could outmaneuver a negotiating opponent in the context of a nuclear confrontation.” Tweet This

While there was much more discussion of the danger of nuclear war during the Cold War, Blair sees the danger as much greater now:

“You might say the temperament and rationality assumption held up, on the whole, on the Soviet side, and for the most part on the U.S. side. Those were the two players, and you could count on their rationality and caution.”

“I think we’re looking now at nine countries with nuclear weapons. Half of them are led by individuals whose temperament and rationality are in question, and that includes Trump.” Tweet This

“There are a lot more leaders with their fingers on the button, and more serious doubts post-Cold War than during the Cold War about whether they would exercise good judgement or whether they would make a bad call. “