As rare bipartisan momentum builds on Capitol Hill for a change to gun laws, the wild card in the debate might be the man in the White House.

President Trump has a long and checkered history on gun reform that stretches back decades and runs a spectrum of conflicting positions, from promotion of an assault weapons ban to a more recent embrace of the Second Amendment protectionism advocated by the National Rifle Association (NRA).

Following Sunday's mass shooting in Las Vegas, the deadliest in the country's history, the president vowed to revisit federal gun laws "as time goes by." That promise will be tested by Democrats, who are pushing hard this week for a range of legislative steps to reduce gun violence.

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It may also prove a headache for GOP leaders, who for decades have aligned themselves with Second Amendment advocates in blanket opposition to new restrictions on buying, owning or operating firearms — a stance they've largely maintained following the horror in Las Vegas.

“It’s premature to be discussing legislative solutions, if there are any,” Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) said Tuesday.

Faced with the Republicans’ refusal to debate the issue, Democrats are increasingly turning to the White House in hopes that Trump will take their side and use his bully pulpit to bring Republicans along.

If recent history is any indication, though, they have their work cut out for them.

Trump, since hitting the presidential trail in 2015, has positioned himself as a fierce champion of gun rights and staunch opponent of any Second Amendment restrictions. During the campaign, he argued that gun violence “has nothing to do with guns,” blaming the problem on gun-free zones like those surrounding schools and “people that are mentally ill.”

“The gun laws have nothing to do with this,” he told ABC’s “This Week” program in October of 2015. “This isn't guns. This is about mental illness.”

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In April, Trump spoke before the NRA’s annual convention — the first sitting president to do so since Ronald Reagan — and vowed he’d never infringe on the right to bear arms.

“You came through for me, and I am going to come through for you,” he said.

Since taking office, Trump has rolled back Obama-era regulations designed to reduce gun violence. And following Sunday’s events in Las Vegas, the president has largely sidestepped the gun-reform debate, focusing instead on memorializing the victims and praising the first responders.

“The president is a strong supporter of the Second Amendment,” White House press secretary Sarah Huckabee Sanders said. “That hasn't changed.”

Citing that history, many Democrats aren’t holding their breath for Trump to back tougher gun laws in response to the Las Vegas massacre.

“The best time to have made that statement would have been when he was in Las Vegas [on Wednesday], and he didn’t,” said Rep. Bennie Thompson (D-Miss.). “Something might come, but if you’re talking about impact, the impact opportunity has come and gone.”

Still, Trump was not always opposed to tougher gun laws.

In “The America We Deserve,” his book published in 2000, when he was exploring a bid for president under the Reform Party, Trump accused Republicans of being pawns of the gun lobby.

“The Republicans walk the NRA line and refuse even limited restrictions,” he wrote.

In the same book, he also promoted an assault weapons ban and “a slightly longer waiting period to purchase a gun.”

More recently, after the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Conn., Trump hailed then-President Obama for launching an aggressive campaign against gun violence that included 23 executive actions and a series of legislative proposals he urged Congress to adopt.

“President Obama spoke for me and every American in his remarks in #Newtown Connecticut,” Trump tweeted at the time.

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The Democrats have been buoyed by a pair of recent agreements — one on the budget, the other on immigration — cut between Trump and Democratic leaders in the face of GOP opposition. They’re hoping that cooperation extends to the gun debate.

“Historically, he was on the other side,” said Rep. Mike Thompson (D-Calif.), the head of the Democrats’ gun-violence prevention task force, who’s urging Trump to join the Democrats on legislation like expanded background checks and a prohibition on gun sales to those on the FBI’s “no-fly” list.

The Democrats may see some rare movement on gun legislation in the coming weeks, as the Vegas shooting has sparked a sudden interest in bump stocks — devices which essentially convert semi-automatic weapons, which are legal, into automatic weapons, which are not.

The devices were used by the shooter in Nevada, likely increasing the lethality of the attack, and several bills banning them have emerged this week. Some lawmakers are also pushing the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to reclassify bump stocks, making them unavailable to most consumers without the need for congressional action.

Sanders, the White House spokeswoman, said Trump is “certainly open” to a discussion on whether bump stocks should be prohibited.

Democrats are welcoming those proposals, along with the willingness of some Republicans to endorse them. But they’re also warning that a bump-stock ban is hardly enough to stop the epidemic of gun violence across the country.

“If that’s the bill in front of me, I’ll vote for that,” Thompson said. “[But] if this majority believes that a bump-stock prohibition is enough, they’ve been smoking something.

“It just doesn’t even come close.”