I suspect what we are seeing here is an inverse of the situation with free trade. On immigration and trade there is less of a Left/Right difference than an Outsider/Insider or Populist/Elite distinction. The Right elites tend to focus on trade to the point where they muffle or suppress much mobilization against this by their own grassroots (e.g., Pat Buchanan). Similarly, Left elites have come to a consensus that populist mobilization by their side against mass immigration is no longer acceptable. And of course no matter the rhetoric, the elites on both sides have traditionally favored the globalist position, though it seems since 2006 the Republican elites have lost control of the immigration issue in their party (though I’m 100% sure that Mitt Romney is simply making populist noises, and will continue with the status quo once on office). Since white liberals fear being perceived as racist (this sentiment was palpable from some of those who supported restrictionism at the conference) this is unlikely a major issue that will come up for them in the near future, and for various reasons the labor wing of the Left coalition no longer emphasizes opposition to immigration. Liberals and Democrats like to contend that the Right has better unity and coherency once a consensus is achieved, with the elite keeping the grassroots in line, but this is one position where for various structural reasons it is the Left which has remained more unified, despite wide ranges of opinions.

Now, log-transformed:

That’s right, TFR, Guatemala (4.15) → Mexico (2.21) → United States (2.05) → Canada (1.53)

(source, 2005-2010 estimates)

Here’s GDP PPP per capita:

United States ($46,400) → Canada ($38,000) → Mexico ($13,600) → Guatemala ($4,800)

This is why the Mexican-Guatemalan border experiences a great deal of traffic.