Saying he's "fairly optimistic about 2010," Paul Krugman says the economy will still need economic stimulus for several years to come and warns Obama to resist the temptation to cut back at the first signs of recovery:

A more plausible route to sustained recovery would be a drastic reduction in the U.S. trade deficit, which soared at the same time the housing bubble was inflating. By selling more to other countries and spending more of our own income on U.S.-produced goods, we could get to full employment without a boom in either consumption or investment spending.

But it will probably be a long time before the trade deficit comes down enough to make up for the bursting of the housing bubble. For one thing, export growth, after several good years, has stalled, partly because nervous international investors, rushing into assets they still consider safe, have driven the dollar up against other currencies — making U.S. production much less cost-competitive.

Furthermore, even if the dollar falls again, where will the capacity for a surge in exports and import-competing production come from? Despite rising trade in services, most world trade is still in goods, especially manufactured goods — and the U.S. manufacturing sector, after years of neglect in favor of real estate and the financial industry, has a lot of catching up to do.

Anyway, the rest of the world may not be ready to handle a drastically smaller U.S. trade deficit. As my colleague Tom Friedman recently pointed out, much of China’s economy in particular is built around exporting to America, and will have a hard time switching to other occupations.

In short, getting to the point where our economy can thrive without fiscal support may be a difficult, drawn-out process. And as I said, I hope the Obama team understands that.

Right now, with the economy in free fall and everyone terrified of Great Depression 2.0, opponents of a strong federal response are having a hard time finding support. John Boehner, the House Republican leader, has been reduced to using his Web site to seek “credentialed American economists” willing to add their names to a list of “stimulus spending skeptics.”

But once the economy has perked up a bit, there will be a lot of pressure on the new administration to pull back, to throw away the economy’s crutches. And if the administration gives in to that pressure too soon, the result could be a repeat of the mistake F.D.R. made in 1937 — the year he slashed spending, raised taxes and helped plunge the United States into a serious recession.

The point is that it may take a lot longer than many people think before the U.S. economy is ready to live without bubbles. And until then, the economy is going to need a lot of government help.