Libertarian Will Wilkinson has been doing some really good posts defending free immigration. I've been puzzled by why the Republicans have been anti-immigration and the Democrats pro-immigration. Immigration boils down to economics. From our point of view, it's a question of cheap labor for consumers vs. higher wages for workers. You'd expect the free-trade folks to be pro-immigration, with the anti-globalization folks being anti-immigration. The moral considerations Wilkinson brings up are also worth considering, of course.