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Adam Posen, the deputy director of the Peterson Institute for International Economics in Washington, said Mr. Obama’s plan could provide just the right boost — if it was carried out properly.

But as the Federal Reserve has been learning for months now, the biggest obstacle to economic activity right now is not a shortage of money. The real obstacle is pervasive fear, which has made banks reluctant to lend and companies reluctant to invest in expansion.

Alan J. Auerbach, an economist at the University of California, Berkeley, said the overall scale of the program looked “reasonable” at $800 billion over two years.

“It’s much bigger than anything that’s been tried in my lifetime, but this is scarier than anything we’ve seen in my lifetime,” Professor Auerbach said.

Left to their own devices, many Congressional Democrats would prefer to focus almost entirely on spending projects and avoid tax incentives.

“One thing we learned from the Depression is marginal, incentive changes don’t work very well when the economy is falling away from you very rapidly,” said Senator Kent Conrad, Democrat of North Dakota and chairman of the Senate Budget Committee. “And that’s what’s occurring here.”

But Republicans have been adamant about the need for tax breaks, and Mr. Obama has made it clear he would like to bring as many members on board as possible.

Representative Paul D. Ryan, Republican of Wisconsin, said in an interview, “I really do believe that if you combine the evidence of history along with the psychological concerns about making investments in the economy today, the better bang for your buck is lower taxes that are certain and permanent and lasting.”

The Democratic plan would direct much of the stimulus money to low-income and middle-income families. That reflects both traditional Democratic concerns about helping lower-income households, as well as the view of economists who say that people with lower incomes are more likely to spend rather than save any money they receive from the government.

Mark M. Zandi, chief economist at Moody’s Economy.com, a forecasting firm, told a forum of House Democrats this week that the “bang for the buck” — the additional economic activity generated by each dollar of fiscal stimulus — was highest for increases in food and unemployment benefits. Each dollar of additional money for food stamps yields $1.73 in additional economic activity, Mr. Zandi estimated, and each extra dollar in unemployment benefits yields about $1.63.

By contrast, Mr. Zandi estimated, most tax cuts produce less than a dollar for each dollar of stimulus, especially if the tax cuts are temporary, because people save at least some of their extra money.

One of the few tax cuts that economists say can generate a positive bang for the buck is a reduction in payroll taxes for Social Security and Medicare.

Mr. Obama wants to offer a tax credit of $500 for individuals, and up to $1,000 for families, which they would receive through a temporary reduction in payroll tax withholdings. The idea, known as the Making Work Pay credit, was part of Mr. Obama’s economic platform during the presidential campaign. As originally envisioned, it would have been available to households with annual incomes as high as $200,000.

But economists said the tax credit could have drawbacks as an economic stimulus measure, mainly because people usually save part of the money or use it to pay down debt. That makes good sense from an individual’s standpoint but does nothing to increase economic activity.

Joel Slemrod, a professor of tax policy at the University of Michigan, said, “The research I’ve done on the 2001 and 2008 tax rebates suggests that the proportion of the rebates that went to spending was rather small, about one-third.”

After Congress approved Mr. Bush’s tax rebate to individuals and families last spring, economic activity jumped fleetingly during the summer, and then stalled out again in the fall.

Some Democratic officials were also skeptical.

“It’s not that rebates don’t work under normal conditions,” said one senior Democratic aide in the Senate. “It’s that current conditions are not normal and are not favorable to rebates or broad tax relief.”