The 2012 presidential election is fundamentally a contest between our future and our past. Barack Obama’s America is the America that will be; Mitt Romney’s is the America that was. And the distance between the two is greater, perhaps, than in any election we’ve had since the Civil War.

The demographic bases of the rival coalitions couldn’t be more different. Monday’s poll from the Pew Research Center is just the latest to show Obama with a decisive lead (in this case, 21 percentage points) among voters younger than 30. Obama’s margin declines to six points among voters ages 30 through 44, and he breaks even with Romney among voters ages 45 through 64. Romney’s home turf is voters 65 and older; among those, he leads Obama by 19 points.

Age polarization is not specific to the presidential election. On a host of issues, as diverse as gay and lesbian rights and skepticism about the merits of capitalism, polls have shown that younger voters are consistently more tolerant and well to the left of their elders.

Nor is age the only metric through which we can differentiate our future from our past. The other is race, as the nation grows more racially diverse (or, more bluntly, less white) each year. While the 2000 Census put whites’ share of the U.S. population at 69.1 percent, that share had declined to 63.7 percent in the 2010 Census, while the proportion of Hispanics rose from 12.5 percent to 16.3 percent. In raw numbers, total white population increased by just 1.2 percent during the decade, while the African American segment grew by 12.3 percent and the Hispanic share by 43 percent. Demographers predict that the white share of the U.S. population will fall beneath 50 percent in the 2050 Census.

Rather than trying to establish a foothold among America’s growing minorities, however, Romney and the Republicans have decided to forgo an appeal to Hispanic voters by opposing legislation that would grant legal status to undocumented immigrants brought here as children and by backing legislation that effectively requires Hispanics to carry documentation papers in certain states. Republicans seek a majority through winning an ever-higher share of white voters. The Washington Post reported last week that its polling showed the greatest racial gap between the Democratic and Republican presidential candidates since the 1988 election, with Romney favored by 60 percent of white voters and Obama by 80 percent of minority voters (a figure that may prove low, if three-quarters of the Hispanic vote goes to Obama, as some other polls suggest it will). The problem for Republicans, of course, is that the minority vote is a far larger share of the total vote today than it was 24 years ago.

By repeatedly estranging minorities and opposing social policies favored by the young, the Republicans have opted for a King Canute strategy: standing on the shore and commanding the tide to stop. Republicans with an eye toward the future, most notably George W. Bush and Karl Rove, have urged the party to embrace immigration reform, but the base is rabidly anti-immigrant and its antipathy is reinforced daily by talk radio hosts and Fox News chatterers who depict an America under siege by alien forces.

Should Republicans prevail in this election and seek to build a more-than-one-term plurality, they will confront a stark choice: Either Romney must persuade his party to reverse its stance on immigration, or the party must seek to extend the scope of its voter-suppression efforts. Put another way, they must try to either accommodate the future or suppress it.

Accommodation with diversity and modernity, however, is simply not part of the Republican DNA. Today’s Republican Party has largely cornered the market on religious fundamentalists, even as the number of GOP scientists has dwindled (a 2009 Pew poll of scientists found that just 6 percent self-identified as Republicans, while 55 percent said they were Democrats). Many of the largest Republican funders come from economic sectors hardly distinguished by significant productivity increases or their contributions to mass prosperity (casino gambling, Wall Street), while Silicon Valley remains more Democratic turf. (By the way, all those messages Republican CEOs have been sending their employees , predicting layoffs should Obama be reelected? Have any of them promised raises if Romney wins? Just askin’.)

Two Americas are facing off in next week’s election. By their makeup, the Democrats are bound to move, if haltingly, into the future, while the Republicans parade proudly into the pre-New Deal past — some of it mythic, lots of it ugly. The differences could not be clearer.

meyersonh@washpost.com