Yesterday’s big news from the US is that John McCain’s campaign has conceded that it is unlikely to hold the state of Colorado — or at least, as one source said, “we need to have a math scenario that doesn’t count on it.” To understand why this is so important, some psephological background is required.

For the last couple of weeks, the mathematics of the presidential election have looked pretty simple. In addition to the states John Kerry won in 2004, Barack Obama needs another 18 votes in the electoral college. Twelve of those look to be sewn up in Iowa and New Mexico, needing two-party swings of only 0.3% and 0.4% respectively.

Beyond that, there are half a dozen large or medium states low down on the Republican side of the pendulum: Ohio (1.1%), Colorado (2.4%), Florida (2.5%), Missouri (3.6%), Virginia (4.1%) and North Carolina (6.2%). Provided he holds all the Democrat states, Obama needs just one of those to win a majority (he’s currently leading in all of them).

McCain needs to hold every one.

So without Colorado, the Republicans have to pick up something from the Democrat side of the pendulum. The obvious target is Pennsylvania on 1.3%, with 21 electoral college votes. If he wins Pennsylvania, McCain could afford to lose not just Colorado but maybe Virginia as well, which most commentators now count as solid for Obama.

Latest polling, however, has Obama even further in front in Pennsylvania than in Colorado or Virginia. If McCain is pinning his hopes on it, it must be that he thinks Obama’s support is softer there, or that there’s something the polls are not (yet) picking up.

That in turn could mean a racially charged end to the campaign, since the racial issue is the thing that’s likely to count more in Appalachia (which, broadly speaking, covers Ohio, Virginia and North Carolina as well) than in the high-tech cosmopolitan world of Colorado. An already ugly contest could get uglier still.

But the Pennsylvania backwoods are also the source of one of the most vivid campaign stories doing the rounds of political websites, in which a voter, after consulting her husband, “brightly and matter of factly” tells a canvasser “We’re voting for the n*gger.”

As’s Sean Quinn comments, “How is John McCain going to win if he can’t win those voters?”