Economics plays some role in this. The apparent downturn in immigration in the past 18 months is surely not unrelated to the recession that began, the National Bureau of Economic Research now tells us, in December 2007. The gaming industry in Las Vegas -- then and for most of the preceding 20 years the nation's fastest-growing metro area -- started declining in 2007, and net immigration to Nevada was down 16 percent in 2007-08 from the 2000-07 levels. And reports are coming in of Latinos leaving town as construction of giant hotels on the Strip is shut down by foreclosure.

But immigration is not just about economics. People move, I have come to think, in pursuit of dreams -- or to escape nightmares. One of those dreams -- home ownership in America -- now seems much less attainable than it did just six months ago, with thousands of foreclosures and with subprime loans to low-income buyers presumably a thing of the past. Meanwhile, birth rates in Mexico and much of Latin America took a sharp turn downward around 1990, which means that those entering the workforce there in years hence will have less competition for jobs -- fewer nightmares.

George W. Bush has said that one of his regrets is that he was not successful in getting Congress to pass a comprehensive immigration law, with legalization, guest-worker and enforcement provisions. If Barack Obama and congressional Democrats seek such legislation, they should keep in mind the possibility that the situation they are addressing may be changing. So should those who oppose such a law.

Since Congress considered and failed to pass a comprehensive law in 2006 and 2007, we have learned that tougher enforcement of existing law is possible and can reduce illegal immigration. Now we face a sharply different economic situation, which is presumably less conducive to immigration. This may make the need for a comprehensive law less pressing and at the same time make it politically more palatable.

Our history is one of great surges of migration, immigrant and internal, which begin without much in the way of warning and which end unexpectedly. It's possible -- not certain, maybe not likely, but possible -- that we're witnessing the beginning of one of those endpoints now.