And both sides are right.
As I recently wrote, Republicans are finished winning normal-turnout elections given the nation's dramatic demographic growth:
Republicans have reason to be freaked out. They continue to lose ground with Latinos, getting blown out 71-27 with one of the few groups to increase their raw vote and their share of the electorate this year. And there is little respite on the horizon—the median age of native-born Latinos is 18. Eighteen! Among all Latinos, including immigrants, it's just 27. That compares with 42 years old among non-Hispanic whites. In fact, Pew estimates that Latinos will double their share of the electorate by 2030, based strictly on birth and death rates.Pew estimates that 66,000 Latinos turn 18 and over 100,000 elderly (and more conservative) Americans die every month. That means that by 2016, there will be about three million more voting-age Latinos and about five million fewer elderly voters. And that's going to be the case whether we get comprehensive immigration reform or not.
Or put yet another way, Texas will be a battleground state by 2024. So math-adept Republicans are correct that they'll be electorally irrelevant unless they can make inroads with the Latino vote. (And let's not forget the Asian vote, considering it's now the fastest growing in the country.)
On the other hand, conservatives are right that legalizing 12 million Latinos would gift Democrats a treasure trove of Democratic voters. As one National Review conservative put it:
Let’s assume that only two-thirds of former illegals become U.S. citizens — that’s 8 million new Americans with the vote [...] Since these voters are poorer and less assimilated than Latinos as a whole, they will likely skew more Democratic than their ethnic fellows. Republicans would be optimistic if they counted on winning more than one-fifth of them — i.e., 1.6 million voters. On a 100 percent turnout, that would give the Democrats a net advantage of 4.4 million votes. On a more realistic assumption that these new voters would have a lower than average turnout — say, 50 percent — that would give the Democrats an net additional 2.2 million votes over Republicans.Those new Democratic voters would accelerate the competitiveness of Arizona, Georgia, and Texas, while padding Democratic advantages in Colorado and Nevada (not to mention California and New Mexico).
So yeah, if I'm a Republican worried about electability, neither option makes me feel any better.
The only difference is that as noted, Republicans are already screwed by the nation's demographic changes. They have two options. They can either solidify their opposition to immigration reform and destroy their ability to ever compete among Latinos, or they can hurt themselves in the short-term and allow reform to happen, but preserve their ability to make gains with Latinos in the mid- to long-term.
Both options screw them, and both options screw them hard. Just one gives them a long-term fighting chance.