President Donald Trump on Monday abruptly announced a suspension of immigration to the US during the coronavirus crisis.

Tucker Carlson has been advocating such a ban on his Fox News show for weeks.

Carlson has often served as a sounding board and champion for Trump's hard-line "America First" policies.

Observers say that with the US economy in crisis, Trump is trying to shore up support from his base.

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President Donald Trump took many by surprise on Monday night with his tweeted announcement of a temporary suspension of immigration to the US in light of the coronavirus pandemic.

Though it was not immediately clear what prompted the announcement — which was vague on details — the Fox News host Tucker Carlson, who also serves as an informal presidential adviser and ardent supporter of Trump's "America First" policies, had been advocating such a ban on his show for weeks.

"Certainly the current president has the power to cut legal immigration in order to protect the nation, and, in fact, American law gives him explicitly this right," Carlson told his viewers on April 1, according to Media Matters, a group that monitors conservative media.

"The time to placate corporate interest looking for low wages at all costs has passed — we need to stop this," Carlson added.

Last week Carlson also invited Jeff Sessions — Trump's former attorney general who is campaigning to win back his Senate seat in Alabama — onto his show to argue for the ban.

—TV News HQ (@TVNewsHQ) April 17, 2020

"I am calling on the federal government to immediately halt immigration to the United States until Americans are BACK TO WORK!" Sessions tweeted after his appearance. "It is insanity for this nation to invite foreigners to come in and take any of the few jobs available during this crisis."

Sessions was the earliest congressional champion of Trump's hard-line anti-migrant policies, but he fell out of favor with the president after recusing himself from the investigation into the 2016 Trump campaign's links to Russia, a move that Trump said allowed for the appointment of a special counsel.

Hours after Trump tweeted his announcement on Monday night, Sessions appeared to take a victory lap, congratulating the president on his decision and quoting a Politico report on his appearance on Carlson's show as one of the likely factors underlying it.

Trump's announcement Monday even appeared to surprise at least some members of the Department of Homeland Security — which oversees immigration into the US — with the president having made no mention of such a plan in his daily coronavirus press briefing earlier that day.

In his tweet, Trump cited a desire to "protect the jobs" of US citizens as part of his rationale, though some noted that curbing immigration would also limit the ability of companies to hire from abroad to gear up growth after lockdowns end.

And it is not entirely clear exactly who the new travel restrictions would apply to — as Trump has already suspended all visa applications and extended border restrictions with Canada and Mexico.

Observers say Trump is desperate to shore up the support of his base as his poll ratings plummet and with the 2020 election only months away.

It wouldn't be Trump's first time pursuing policy known to be favored by Carlson and fellow migration opponents.

Carlson was thought to have played a key role in persuading Trump to start taking the coronavirus outbreak seriously in March. Trump has also sought Carlson's counsel in foreign policy.