Had the president achieved anything less than those crushing majorities among non-white voters, had his support among minorities resembled the “normal” levels that characterized nearly all his Democratic predecessors, the outcome of the election could have been profoundly different. John Kerry, the last candidate of the party’s pre-Obama era, received 88% of the black vote, 53% of Hispanics, 56% of Asians and 54% of those who identified themselves as “other.” With these margins in 2012, Mr. Obama would have lost the popular vote. More remarkably, he would have lost decisively in the Electoral College. If Mr. Romney had earned his national percentage of the white vote (59%) with white California voters—and if he had secured the same percentages among blacks, Latinos and Asians in California that George W. Bush won nationally in 2008—he would have carried the Golden State in 2012.

In this scenario, Mr. Romney’s margin in the most diverse major state in the country would have been razor thin—49.15% to Mr. Obama’s 48.25%—assuming that 2.6% of votes still went to minor party candidates. Still, the lesson of this exercise should concern Democrats who assume their unshakable strength with minorities assures future victories. Without Mr. Obama’s overwhelming and unprecedented margins among non-whites, even the bluest of blue states could have been in play. Without his powerful race-based appeal, a future Democratic nominee might fall back to a less lopsided share of the nonwhite vote, following partisan patterns established over the course of a quarter century.

With no special appeal to minorities in 1996, Bob Dole won 12% of the black vote. This was twice the share won by either of Mr. Obama’s Republican rivals—even though Mr. Dole’s opponent, Bill Clinton, had been celebrated throughout his career for his popularity among African-Americans. A competitive Republican candidate need only equal the performance among minorities of such conventional standard bearers as Messrs. Bush and Dole, still ceding Democrats two-thirds (or less) of the growing, nonwhite segment of the electorate. Without a barrier-breaking black candidate, and without the three-fourths of voters of color that Mr. Obama delivered in 2008 and 2012, the Democratic electoral edge all but disappears in 2016.