David Leonhardt has a good analysis of the history of immigration as a political issue in the United States. An excerpt:

Immigration has a fantastically complicated political history in the United States. It has produced enough populist anger to elect Know Nothing mayors of Chicago, Philadelphia, Washington and San Francisco, all in the 1850s and, more recently, to help Lou Dobbs reinvent his television career and become a best-selling author. But when national politicians have tried to seize on such anger, they have usually failed — and failed quickly. “While immigration has always roiled large sections of the electorate,” said Eric Rauchway, a historian at the University of California, Davis, “it has never been the basis for a national election, one way or the other.”

That appears to be truer than ever in 2008. Mr. McCain will all but clinch the Republican nomination on Tuesday with victories in the Ohio and Texas primaries. In the Texas campaign, except for a couple of obligatory questions about a border fence during a Democratic debate, immigration has been the dog that didn’t bark. The candidates who would have made an issue of it exited the race long ago.

There is, however, one more historical parallel to consider: as a political matter, immigration probably won’t go away on its own. The anti-immigration movements of the past may not have created presidents, but they did change the country. The Chinese Exclusion Act helped cut the immigration rate by more than 40 percent at the close of the 19th century. The Nativist movement of the 1910s and 1920s had even more success passing laws to reduce the flow.

I think the reason it fails as a political issue, but not as a matter of policy, has always been about the inherent decency of the American voter. The bigots and xenophobes who are most obsessed by the "illegals" can stir up a hornet's nest for awhile, but as with the issue of welfare reform thirty years ago, it's hard to get a voting majority behind candidates motivated to do something about it. It wasn't Ronald Reagan and his racist talk of "welfare queens" driving Cadillacs that changed the system; it was Bill Clinton and a generation of neo-liberals who approached the issue pragmatically, disassociating the debate from its genesis as a wedge issue.

The same thing, I believe, will ultimately happen with immigration. The nation is entitled to have control over its own borders, and to establish a policy concerning who may enter the country. But voters tend to recoil from the ugly language that is often used in the debate, with "illegals" replacing "wetbacks" as the code term du jour. Immigration reform will only come when the wonks make their voices predominant on this issue.