Whatever your position on immigration reform, the reason that it has support among the Democrats is because of the votes it will garner them. Although the Democrats, by dint of offering lots of free stuff and an election win based on corruption, squeaked by on the last presidential election. They potentially could win the next two or three election cycles. However, demographics suggests that Democrats will lose in the long run simply because they reproduce at a lower rate than conservatives.
They hope to reverse this trend, and solidify their political control, by bringing in millions of new voters that are almost assuredly going to vote Democratic. And the prize is Texas, and its 38 electoral votes. As this New York Times blog notes:
Of course, why bother increasing their voter turn-out if you can simply increase the number of voters?
Regardless of who gets hurt in the process, Texas is ripe for realignment. Insofar as demographics count, Texas is on a path to first turn purple and then blue. The question is when. Whites are already a minority of the state’s population, 44.8 percent, down from 52.4 percent in 2000; blacks are at 12.2 percent, slightly up from 12.0 percent; Hispanics are at 38.1 percent, up from 32.0 percent; with Asian-Americans, Native Americans, and others making up the remaining 4.9 percent. Whites are teetering at just over 50 percent of Texans eligible to vote; according to demographic projections, they will become a minority of voting age citizens in 2016.What makes Texas such an inviting target for Democratic organizers is that Hispanic turnout in the state is strikingly low, 38.8 percent of those eligible to vote, compared to the national average of 48 percent.
You may ask why someone like McCain, who is ostensibly Republican, would be in favor of immigration reform? I suspect the simple reason is that McCain's authority in the Senate rests on his alleged ability to work across the aisle. So, if he can get into a permanent minority, it increases his political capital.