I recently argued that an increased deficit among the white working class could sink Obama’s re-election chances. That’s particularly the case if Obama is already weakened by relatively poor support from his base. Right now, that looks like a distinct possibility.

This can be seen by looking at Obama’s two key base demographics: the minority vote and the youth vote. Start with minorities, the heart of the Obama coalition. In 2008, Obama received 80 percent support from minorities, who were 26 percent of all voters. Can he replicate that performance in 2012?

Consider first the probable minority share of the vote in 2012. Recently released data from the 2010 Census have underscored just how fast the minority population is growing in the US. Over the last decade, the minority population increased by 30 percent (Hispanics alone grew by 43 percent), while the white population grew by a mere 1 percent. Because of this dramatic difference in growth rates, minorities accounted for virtually all (92 percent) of the country's population growth over the decade. And the overall minority share of the population ticked steadily upward while the white share declined. The 2010 minority share of the population was 36 percent, up more than 5 percentage points over 2000. That's a rate of increase of around half a point a year over the decade.

Applying that rate to the four years between 2008 and 2012 indicates that the minority share of voters should be about 28 percent in 2012. Of course, that rate is based on the overall minority population not minority voters. Should the trend rate of increase be lowered to account for this difference? No, if anything it should be increased. Exit poll data show minority vote share increasing at a faster rate last decade than overall population growth, so a 2 point estimated trend increase in minority vote share may actually be conservative.

So if the electorate stays on trend, the minority share of voters in 2012 should not only match but exceed its share in 2008, significantly helping Obama’s re-election bid. But will these voters show up in their expected numbers? That is where Obama could run into trouble. The real guarantor that they will is voter enthusiasm. Lacking that, the expected increase in minority vote share might well not appear. After all, in the 2010 election, when enthusiasm was not high, minority vote share dropped to 23 percent, a poor performance even for an off-year election.