Liberals and conservatives have different ideas of what “reform” means. Liberals think it means legalizing the undocumented, while conservatives think it means securing the borders. Liberals are reluctant to confront labor unions that oppose guest workers, just as conservatives never miss a chance to weaken employer sanctions opposed by big business. But another problem is that, once in power, both sides have the same tendency to overreach. The left should concentrate on what it can achieve in immigration reform and be careful not to repeat the mistakes made by those on the right. Otherwise, they might find themselves with the...