For the first time since Aug. 28, the FiveThirtyEight model projects Mr. Obama to win fewer than 300 electoral votes. But Mr. Biden may be able to do something about the yawning gap in enthusiasm between Democratic and Republican voters. That gap might be the greatest threat to Mr. Obama’s winning another term in the White House, and it seems to have widened since the Denver debate.
Republicans normally turn out at higher rates than Democrats, in part because they tend to be older, whiter and have other characteristics that correlate with voting on a regular basis. But the enthusiasm gap is usually small in presidential election years, when about 75 percent of registered voters turn out. In the presidential elections since 1992, polls of so-called likely voters have typically been one or two points more Republican-leaning than those of registered voters. ...Nate Silver, 538
From Real Clear Politics