President Trump is set to unveil executive actions on immigration—one of his central campaign promises—and could move to suspend some immigration from Iraq, Iran, Libya, Somalia, Sudan, Syria, and Yemen, either temporarily or “indefinitely,” according to sources familiar with the plans that have been under consideration.

The sources also expect that any impending Trump action could also impact refugees from countries with Muslim populations. Trump proposed a ban on all Muslim immigration on a temporary basis early on in the campaign, but has since amended his position.

Although Trump could face legal challenges to these measures the orders will be well within his legal rights as President. Trump is expected to visit the Department of Homeland Security on Wednesday as he prepares to unveil his first executive actions on immigration, according to congressional and administration sources.

After the visa suspension is in place, work on the vetting process will begin. But White House spokesman Sean Spicer said that the nominee for Secretary of State, Rex Tillerson, would need to be installed first.

On Tuesday night, Trump tweeted that his plans for Wednesday centered on national security and one of his main campaign vows, building a wall on the southern border.

President Trump will begin cracking down on border security during a trip to the Department of Homeland Security on Wednesday. He called it a “big day” for national security.

He is expected to sign an executive order to kick-start the construction of the border wall with Mexico. President Trump initially said Mexico would pay for the wall, before backtracking and saying the country would reimburse the US.

He is also likely to order America’s so-called sanctuary cities to cooperate with authorities to deport illegal immigrants if they want to protect their funding.

During the 2016 election, immigration became one of the Trump campaign’s major talking points, with the then-candidate proposing a “total and complete shutdown” of Muslims entering the U.S.

“On Day One, I’m going to begin swiftly removing criminal illegal immigrants from this country,” Trump said at a rally in Des Moines, Iowa, in August. “I promise you, from the first day in office — the first thing I’m going to do, the first piece of paper that I’m going to sign is we’re gonna get rid of these people … Day One.”