At National Journal, Ronald Brownstein runs the numbers on race and the presidential vote:
For President Obama, the winning formula can be reduced to 80/40. In 2008, Obama won a combined 80 percent of the votes of all minority voters, including not only African-Americans but also Hispanics, Asians, and others. If Obama matches that performance this year, he can squeak out a national majority with support from about 40 percent of whites—so long as minorities at least match the 26 percent of the vote they cast last time.
Obama’s strategic equation defines Mitt Romney’s formula: 61/74. Romney’s camp is focused intently on capturing at least 61 percent of white voters. That would provide him a slim national majority—so long as whites constitute at least 74 percent of the vote, as they did last time, and Obama doesn’t improve on his 80 percent showing with minorities.
These calculations underscore the depth of racial polarization shadowing this election and the achingly slim margin of error facing each candidate. If Obama nudges past 80 percent among minorities (which seems very possible) or the minority vote share rises (also possible, though less probable), the president could gain reelection while winning only about 38 percent of white votes. Conversely, if the white proportion of the vote increases just a single percentage point (to 75 percent), and Romney records any gains among minorities, he could shave his winning number to a more manageable 59 percent of whites.