Republican presidential candidate, former Massachusetts Gov. Mitt Romney greets supporters at a campaign rally at Tri-City Christian Academy in Chandler, Arizona, Wednesday, Feb. 22, 2012. (AP Photo/Gerald Herbert) (National Journal)

The dispersion of diversity to new places, even if slowed by the economic downturn, remains one of the defining demographic trends of contemporary America. But even as racial boundaries are falling in community life, they are hardening in our political alignment. That's an ominous prospect.

As Maribel Hastings shows in this issue, the steady growth of the minority population is not only reshaping big cities like Los Angeles and Miami, it's also spilling over into Midwestern small towns less familiar with new faces. Hispanics are the central engine of this change. From 2000 to 2010, the Hispanic population at least doubled in 107 of the nation's 366 largest metropolitan areas, according to calculations from census data by Brookings Institution demographer William Frey. Many of those increases are from low numbers, but Hispanics now constitute at least one-fifth of the population in nearly one-fifth of those metropolitan areas. A transformation that big will leave few places, or institutions, untouched.

So far, however, the GOP's voter coalition is a conspicuous exception. Republicans in 2010 elected several high-profile minority candidates (led by Govs. Nikki Haley of South Carolina, Susana Martinez of New Mexico, and Brian Sandoval of Nevada, as well as Sen. Marco Rubio of Florida) who might eventually expand the party's appeal in nonwhite communities. But today, the Republican Party remains almost entirely dependent on white voters. In the GOP presidential nominating race now concluding, whites cast at least 90 percent of the votes in 17 of the 20 states for which exit polls were conducted.

Overall, in a nation where minorities now comprise more than 36 percent of the population, whites have cast 93 percent of the votes in this year's GOP primaries so far, according to a National Journal compilation of results. That's even more than their 89 percent share in the 2008 GOP primary race, according to a 2008 analysis by Gary Langer, a pollster for ABC. By contrast, whites delivered only 65 percent of the total votes in the 2008 Democratic presidential nomination race, Langer calculated.

The color divide has been every bit as sharp in recent general elections. In 2008, then-Sen. Barack Obama became the first nominee to lose white voters by double digits (Republican nominee John McCain carried them 55 percent to 43 percent) and win the White House; Obama did with the backing of fully four-fifths of minority voters. Put another way, in 2008, whites provided 60 percent of Obama's votes, and minorities 40 percent. Nearly nine in 10 of McCain's votes came from whites.

This racial divide isn't unique to Obama: No Democratic presidential nominee since incumbent Lyndon Johnson in 1964 has carried most whites. But the racial chasm in American politics appears to be deepening. The complex emotions that Obama evokes as the first African-American president undoubtedly intensify this division. Yet it preceded him and will outlast him. The most important factor is probably the divergence between minorities who generally believe that government investment will expand their opportunities and the growing resistance to government activism among whites (especially older and blue-collar ones). Hard times are sharpening that disagreement.

Attitudes toward the demographic change itself are also contributing. In the first University of Phoenix/National Journal Next America Poll conducted in April, whites split nearly in half over whether the United States has made the changes necessary to ensure equal rights for all races. Whites who said that more changes were needed backed Obama over Mitt Romney by 3-to-2. Those who said enough changes had already been made preferred Romney over Obama by more than 2-to-1. That doesn't mean racism is driving opposition to Obama. But it does suggest that attitudes toward demographic change now reinforce the ideological and cultural differences separating the two parties' coalitions.

Romney can win the White House by consolidating slightly more whites than Republicans won in the 2010 midterm. That's a steep, but not impassable, hill. Over time, though, as the minority share of the vote rises and diffuses, Republicans will need to attract an implausibly high percentage of whites to win presidential elections if they cannot dent the Democratic dominance among nonwhites.

The larger issue is whether an increasingly diversifying nation can forge the consensus it needs to move forward on its biggest challenges if racial differences reinforce its partisan polarization. As minorities spread into more communities, the incentive could rise for both parties to compete for their votes. The alternative is hardening racial division as the growing visibility of minorities drives more whites who are uneasy about the change toward the GOP. America is unlikely to find common cause on any of its biggest needs if blue versus red becomes just a synonym for black and brown versus white.

Ron Brownstein is National Journal's editorial director.