WASHINGTON — President Trump has spent the past several weeks dismissing concerns about any economic effects from the coronavirus and insisting that American companies and consumers are in no danger.

On Monday, after stock markets suffered their worst single-day decline in more than a decade, Treasury yields reached new lows and oil prices plummeted, Mr. Trump said he was ready to act.

The president said the White House would unveil “very dramatic” proposals to help protect the American economy from the coronavirus, including financial help for workers who don’t get paid sick leave, tax cuts and targeted relief for industries battered by the virus, including cruise lines, airlines and hotels.

The overture sets up tense negotiations with Congress, just weeks after the president was impeached by House Democrats and less than a year from Election Day. Democrats remain deeply critical of Mr. Trump’s efforts to contain the virus and have suggested, at least for now, that they will move cautiously to help bail out the president’s handling of the crisis.