We can debate whether those remaining undecideds, ranging from 3 to 8 percent in most of these polls, will break heavily for the challenger. In 2004, George W. Bush and John Kerry split the remaining undecideds roughly evenly. But the one scenario that political scientists deem virtually impossible is one where undecideds who have declined to support the incumbent all year suddenly break heavily in favor of him. For most of the remaining undecideds, the choice is between voting for the challenger and staying home.

The polling currently suggests President Obama has a hard ceiling of about 47 percent, perhaps 48 percent. Let’s take the 50-47 split found currently in the Rasmussen, Washington Post, and Gallup tracking polls. Presume that most of the remaining undecideds stay home and that the vote for third-party candidates amounts to about a percentage point. Under that scenario, we would see a 51 percent to 47.9 percent popular vote win for Romney.

There are two other little-discussed indicators pointing to a Romney popular vote win – the GOP challenger’s level of support in the uncontested blue states and the uncontested red states.