As immigration reform grinds its way through the U.S. Senate, the main focus has rightly been on the legislation's policy consequences. But there are important political implications, especially for the GOP, that are worth examining.

Some observers, including Phyllis Schlafly, Pat Buchanan and the Center for Immigration Studies, argue that if Republicans want to win back the White House, they should focus on white voters (who comprised 72% of the electorate in 2012) rather than worrying about Latinos. After all, new Census Bureau estimates are that 100,042,000 whites voted in 2008 but only 98,041,000 did in 2012. Wouldn't it be better to get those two million whites back into the polling booth?

This argument is incomplete. If as many white voters turned out in 2012 as in 2008—and if Mitt Romney received the 59% of them that he got last fall—then his vote total would have increased by 1,180,590. But President Obama's vote total would have increased by 780,390, and Mr. Romney still would have lost the election by 4.6 million votes.

To have prevailed over Mr. Obama in the electoral count, Mr. Romney would have had to carry 62.54% of white voters. That's a tall order, given that Ronald Reagan received 63% of the white vote in his 1984 victory, according to the Congressional Quarterly's analysis of major exit polls. It's unreasonable to expect Republicans to routinely pull numbers that last occurred in a 49-state sweep.

Moreover, a Reagan-like percentage of white voters would yield a much narrower win today. That's because the nonwhite share of the vote had doubled to 28% in 2012 from 13% in 1984, according to national exit polls.