In the long run, the most powerful demographic trends will continue to benefit Democrats, at least at the presidential level. But that advantage will be offset if Democrats can’t sustain more support from whites, especially when the party holds unified control in Washington and can implement its agenda, as Obama did during his first two years in the White House. Democrats may now need fewer whites to win a national majority, but Obama is laboring to clear even that lowering bar, especially since the surge toward Romney after the first presidential debate.

In combination, the stark class, generational, and—above all—racial fissures now shaping American politics have produced a closely divided and volatile electorate that has stubbornly refused to provide either party with a lasting advantage, arguably since the Reagan era, and voters next week may again divide almost exactly in half. “A political system in which one party is the party of white America and the other party is the party of minority America is a political system that is structured for polarization,” says veteran Democratic analyst William Galston, a senior fellow at the Brookings Institution. “It’s a political system that locks us into an endless continuation of what we’ve seen.”