In three news conferences last week, President-elect Barack Obama began to outline an economic stimulus and recovery program involving public works, tax breaks and new federal funding for energy research.

The planned initiatives provided a spark of optimism amid the worsening U.S. financial outlook, but elements crucial for a sustained recovery are yet missing and many important details remain to be filled in, economists say. These include Obama's approach to mortgage relief and the structure of a bailout of the automobile industry.

Some of the gaps may be deliberate, as insiders debate the size and scope of the package. Most economists argue that to be effective, the program will have to be massive.

"The most important elements will be size and duration — $300 (billion) to $500 billion and two years sounds right," said Jared Bernstein, an economist with the Economic Policy Institute.

Others say that placing a dollar figure on the program now, while the magnitude of the economic slowdown is still unknown, may only hamstring the new administration.

"He's wise not to put a figure on it yet," said Robert Kuttner, founding co-editor of the American Prospect, a liberal journal, and an informal adviser to Obama's electoral campaign. "Six weeks from now, it might be a bigger number."

Conservative economists, meanwhile, say Obama is planning to do too much, not too little.

"Whatever federal spending is included in the package eventually will have to be paid for by raising taxes, printing money or additional borrowing, all of which drags on the private economy," William F. Shughart II, a former economist at the Federal Trade Commission and professor of economics at the University of Mississippi, said in an e-mail.

But mainstream economists tend to argue that battling a major slowdown demands that the government temporarily step in, with deficit spending if necessary, to substitute its own resources for evaporating private investment.

Obama has said little about how he intends to address the housing and mortgage crisis — an indispensable fix for a sustained recovery, economists say.

Another issue on which Obama has not taken a detailed stand is the fate of the U.S. automobile industry, which is seeking government aid to help it survive the sharp drop-off in sales created by the evaporation of credit and the economic slowdown.

Obama has backed relief for the Big Three in principle, but has said automakers must come up with "a better thought-out proposal" to use federal funds when they come before Congress with a new request this month.