Real estate prices slid in just about every part of the country in December, pushing a housing market that once seemed to be rebounding nearly back to its lowest level since the crash began.

At this dismal point, some economists and analysts say that the damage has been done, and there is nowhere to go but up. Many others argue that the market has still not finished falling.

And then there are those who maintain that, possibly, things are about to get a whole lot worse.

Robert J. Shiller, the Yale economist who is the author of “Irrational Exuberance” and who helped develop the Standard & Poor’s/Case-Shiller Home Price Index, put himself in this last group. Mr. Shiller said in a conference call on Tuesday that he saw “a substantial risk” of the market falling another 15, 20 or even 25 percent.

The 20-city Case-Shiller composite is already off 31.2 percent from its peak, according to data released Tuesday. Average home prices in Atlanta, Cleveland, Las Vegas and Detroit are below the levels of 11 years ago. A drop the size that Mr. Shiller says he thinks could happen would put Chicago, Dallas, Charlotte and Minneapolis there, too. It would create a lost decade for housing in much of the country even before the effects of inflation.