Donald Trump walks from Marine One of the White House in Washington after returning from Florida. AFP

Concern that President Donald Trump himself could be exposed to the coronavirus through contact with two Republican lawmakers loomed Monday as US stocks plummeted, feeding growing national anxiety.

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Trump, who flew back to Washington after a weekend golfing at his Florida resort and having dinner with Brazil's right-wing president, has spent weeks dismissing the seriousness of the threat.

Nurse Becky Barton gives nurse Jess White some hand sanitizer as she changes her protective gear after testing a patient at a drive-through clinic for coronavirus in Seattle, Washington. Reuters

But after an emergency meeting with administration staff, he told journalists that he would propose "very substantial" economic measures to Congress on Tuesday. These will include tax relief and aid for workers in the gig economy who worry about calling in sick.

Trump said he would hold a press conference to discuss the measures on Tuesday, adding the coronavirus — which has killed 26 people in the US — has "blindsided the world."

The run on stock market prices robbed Trump of one of his main talking points ahead of the November presidential election. The Dow Jones closed 7.8 percent lower in the worst session since 2008.

And with pictures of a virus-hit cruise ship constantly on live television, Trump's tweet Monday that "life & the economy go on" seemed increasingly out of touch.

Ambulances are seen in a staging area at the Life Care Center for coronavirus cases in Kirkland, Washington. Reuters

Alarm mounted further after at least two Republican lawmakers who recently met with the president announced they were going into self-quarantine, fearing that they were exposed to the virus at a conservative conference just outside Washington.

One of them, Representative Matt Gaetz, had traveled with Trump on Air Force One on Monday.

Another, Representative Doug Collins, was with Trump Friday during a coronavirus briefing at the Centers for Disease Control and Prevention headquarters.

A patient is loaded into an ambulance at the Life Care Center in Kirkland, Washington on Monday. AP

Trump's incoming chief of staff, Mark Meadows, also went into self-quarantine Monday after learning he may also have been exposed at the same conference.

Although he is not exhibiting symptoms, and a precautionary test came back negative, "he'll be self-quarantined till the 14 day period passes Wednesday," his spokesman said on Twitter.

The spreading virus also raises questions over whether Trump will be able to continue holding the large rallies at the heart of his reelection campaign.

Agence France-Presse