President-elect Donald J. Trump confirmed on Monday that he intended to declare a national emergency and use the U.S. military in some form to assist in his plans for mass deportations of undocumented immigrants.

On his social media platform, Truth Social, Mr. Trump responded overnight to a post made earlier this month by Tom Fitton, who runs the conservative group Judicial Watch, and who wrote that Mr. Trump’s administration would “declare a national emergency and will use military assets” to address illegal immigration “through a mass deportation program.”

At around 4 a.m., Mr. Trump reposted Mr. Fitton’s post with the comment, “TRUE!!!”

Congress has granted presidents broad power to declare national emergencies at their discretion, unlocking standby powers that include redirecting funds lawmakers had appropriated for other purposes. During his first term, for example, Mr. Trump invoked this power to spend more on a border wall than Congress had been willing to authorize.

In interviews with The New York Times during the Republican primary campaign, described in an article published in November 2023, Mr. Trump’s top immigration policy adviser, Stephen Miller, said that military funds would be used to build “vast holding facilities that would function as staging centers” for immigrants as their cases progressed and they waited to be flown to other countries.

The Homeland Security Department would run the facilities, he said.

One major impediment to the vast deportation operation that the Trump team has promised in his second term is that Immigration and Customs Enforcement, or ICE, lacks the space to hold a significantly larger number of detainees than it currently does.

That has sometimes led to allowing asylum seekers into the country while they await court dates with immigration judges, a practice critics deride as “catch and release.”