U.S. President Donald Trump and Vice President Mike Pence participate in a coronavirus briefing with health insurers in the Roosevelt Room at the White House, on March 10, 2020 in Washington, DC.

The speed with which the White House and Congress can agree a stimulus package to fight the impacts of the coronavirus pandemic is nearly as important to markets as the actual financial details.

For now, some economists expect the impact on economic activity as the U.S. fights spread of the virus could result in a period of flattish and even slightly negative growth, but they say there are much deeper risks to the economy if the fiscal stimulus does not come soon. At the same time, the Fed is expected to continue easing, and some forecasters expect the central bank to take its target federal funds rate range to the zero bound within the next two months.

"You want to try to get stimulus into the hands of people and businesses quickly because the shock is hitting extremely quickly," said Bruce Kasman, J.P. Morgan chief economist and global head of economic research. "You want to see it targeted to industries and workers being hurt. If you believe the shock is likely to fade quickly, as we do…you also want whatever you do to not have a lasting footprint. That's not what's required or at least what's appropriate."

President Donald Trump was expected to address the nation Wednesday evening on the U.S. response to the virus. So far, there is no clearly defined stimulus package from the White House, though Trump administration officials say a package will be coming soon.

Trump has said he would like to eliminate the payroll tax for businesses and employees for the rest of the year. Elimination of that tax would amounts to about 1.7% of GDP, according to Strategas Research.

But there is some skepticism that a bi-partisan deal can come together quickly, and Democrats in the House have already pushed back on the idea of the payroll tax holiday.

A show of unity in Washington would help boost confidence. Kasman said it would be helpful to see that Fed Chairman Jerome Powell was working with Treasury Secretary Steven Mnuchin, as Fed Chairman Ben Bernanke did with Treasury Secretary Hank Paulsen in the early stages of the 2008 financial crisis. But the Fed has taken steps already, and the action needed now is fiscal in nature.

"The question is in the U.S., is that a job for the Fed or a job for the Treasury? Right now, it is more a job for the Treasury," said Kasman. "It's probably more appropriate to channel money through the Small Business Administration, in terms of lending, to do things in terms of the tax code or in loan guarantees that help the airline or hotel industry….Ultimately, if we get into much bigger trouble, we can talk about the Fed and Treasury doing some of the things they did during the financial crisis. Its' premature to talk about that."

On the monetary track, J.P. Morgan economists expect the Fed to cut interest rates by a full point, back to the target range of 0 to 0.25%, when it meets next week. Other forecasters expect a half percent cut now and a move to zero a at the April meeting, while the futures market is currently priced for a cut of 0.75% next week.

The Fed already shaved a half percent off the rate in an emergency move last month, and on Wednesday it again expanded its repo operations to assure plenty of liquidity for the short term lending market.

But the package of stimulus from the federal government is now what could make the most impact and provide some psychological support for markets. The Dow was down 1,600 points Wednesday, as Wall Street awaits a clear response from Washington, amid headlines the virus is spreading and that the World Health Organization has now declared it a pandemic.

"You want to see the government to be decisive, not dismissive," said Sam Stovall, chief investment strategist at CFRA. Stovall said the package could give the market some support, but it will not stop the selling. What stimulus would do is help the market recover once it hits bottom.