Take the map we just drew and make two small alterations: Mr. Obama holds onto Nevada, but loses Pennsylvania. (Seem implausible? It isn’t: Mr. Obama carried Nevada by a wider margin than Pennsylvania in 2008.)

Under the old rules, this would be enough for the Republican candidate to become the next president, winning the Electoral College 273-265. If Pennsylvania’s votes were split, however, Mr. Obama would be very likely to carry at least five of them in Philadelphia and Pittsburgh. That would be enough for him to win the Electoral College 270-268…

I don’t entirely buy that President Obama has problems with his base — his numbers are still pretty decent with most Democratic core voters. But they have eroded some, even among African-Americans. And even if these voters still support Mr. Obama, they may not turn out, or they may turn out but decline to devote their time and money to his campaign.

I can’t think of many better ways to motivate these voters than to convince them that Republicans are trying to steal the election, and remind them of what happened in 2000, themes that will become prominent should the Pennsylvania plan come to pass.