Washington CNN  — 

President Donald Trump announced on Friday that his administration will issue a new rule banning bump fire stocks, a gun accessory that makes it easier to fire rounds quickly from a semi-automatic weapon, mimicking automatic fire.

The announcement comes a day before large numbers of protesters are expected to descend on Washington, DC for “March for Our Lives,” a rally organized to push for stricter gun laws in the wake of the school shooting in Parkland, Florida last month.

“Obama Administration legalized bump stocks. BAD IDEA. As I promised, today the Department of Justice will issue the rule banning BUMP STOCKS with a mandated comment period,” Trump wrote. “We will BAN all devices that turn legal weapons into illegal machine guns.”

Shortly after Trump’s tweet, Attorney General Jeff Sessions announced that the Justice Department would clarify rules that define bump stocks within the definition of “machine gun” under federal law, a move that would ban the sale of such accessories.

Trump directed his attorney general to propose changes that would ban bump stocks in February. The move came four months after a gunman in Las Vegas used a bump stock to fire down on concertgoers, killing 58 people, the deadliest mass shooting in modern American history.

Republicans, in response to the Las Vegas shooting, blamed the Obama administration for the sale of bump stocks.

“Just a few moments ago I signed a memo directing the attorney general to propose regulations that ban all devices that turn legal weapons into machine guns,” Trump said at a Medal of Valor event at the White House in February.

“I expect these regulations to be finalized, Jeff, very soon,” Trump added.

On Friday, a Department of Justice statement said if the rule change is made, bump stocks and similar devices would be “required to surrender, destroy or otherwise render the devices permanently inoperable.” The public comment period on the rule is 90 days.

“After the senseless attack in Las Vegas, this proposed rule is a critical step in our effort to reduce the threat of gun violence that is in keeping with the Constitution and the laws passed by Congress,” Sessions said.

This announcement is the near culmination in a process that has been ongoing for months. The initial proposal by the Bureau of Alcohol, Tobacco, Firearms and Explosives drew more than 35,000 comments from the public, a number far more than usual, a sign of how contentious the issue has been.

White House press secretary Sarah Sanders said at the time that Trump “doesn’t support use of those accessories.”

Though bump stocks were not used in the shooting at Marjory Stoneman Douglas High School, they were one of the proposals Trump backed in the wake of the deadly shooting.

The President, in a series of emotional events, also proposed arming teachers and school staff – a suggestion that was quickly slammed by pro-gun control and education groups – and making it easier for judges to rule someone unfit to own a gun.

Trump will not in be in Washington on Saturday during the march, organized in part by students from the Parkland, Florida high school, to protest US gun laws and school shootings.

Some lawmakers have questioned whether a rule through the ATF goes far enough to banning certain gun accessories.