Economists are now predicting a severe slowdown, with growth contracting sharply and a rise in unemployment that could reach 15 percent by midyear. Mr. Mnuchin and Mr. Powell’s efforts are critical not only to workers and businesses but also to Mr. Trump, who has staked much of his reputation on a climbing stock market and robust economy, both of which are now threatened as he faces re-election.

“I wish we could have our old life back. We had the greatest economy that we’ve ever had,” Mr. Trump said at a White House briefing on Sunday. “We are bringing our economy back strong like it was before. It is even better than before. A lot of the money you read about, that’s all coming back.”

Mr. Mnuchin and Mr. Powell have been rapidly deploying resources to try to ensure that rebound happens. Mr. Mnuchin worked on details of the new legislation with Congress, pushing for money that could be used to aid companies big and small. Mr. Powell and other Fed officials, including Randal K. Quarles, the vice chair for supervision and regulation, talked with lawmakers about the Treasury Department funding to backstop the Fed’s programs.

Mr. Mnuchin now has enormous influence in doling out funds to hard-hit industries and small businesses, and the Fed’s emergency lending programs will be pumped up with a $454 billion taxpayer investment — enough to insure against losses on what could amount to more than $4 trillion in lending.

“It is shaping up as a power move with quite a bit of discretionary and open-ended decision making authority being consolidated within the Federal Reserve and the secretary of the Treasury,” said Sarah Bloom Raskin, a former governor of the Federal Reserve Board who was deputy Treasury secretary in the Obama administration.

Besides backstopping the Federal Reserve’s lending facilities, the Treasury Department is helping to manage a $350 billion fund that will provide grants and loans primarily to small businesses.

Mr. Mnuchin will have leeway over another $46 billion in industry-specific funds that Congress has appropriated to help the airline industry and companies involved in national security. He will be able to decide which businesses are worthy of loans and loan guarantees; when interest should be waived; when restrictions on corporate stock buybacks should be lifted; and whether or not to take an equity stake in a company in exchange for bailing it out.