It could all depend on turnout

Obama Would Be Big Favorite With ‘Fired Up’ Base

By NATE SILVER, NYT

There's one advantage that President Obama has that Mitt Romney probably doesn't. If he can get a good turnout from his base, he'll be the heavy favorite to win in November - even if Mr. Romney gets a strong turnout as well.

On average over the last five public surveys, 35 percent of registered voters identify themselves as Democrats and 30 percent as Republicans. That advantage is down somewhat for Democrats since 2008, but it is an advantage nevertheless.

In essentially every recent presidential election, however, the Democratic candidate has performed worse among actual Election Day voters than among the broader pool of registered voters. There is no reason to think that this year will be an exception. Recent surveys that compare likely-voter with registered-voter results suggest that there could be a turnout gap of around three percentage points favoring Mr. Romney. That's larger than the historical average, when it's been in the range of one or two points.

Our election forecasts build in a likely voter adjustment for this reason. If a pollster publishes both registered-voter and likely-voter results, we use the likely-voter version of their numbers. And if only a registered-voter version is available, we shift the numbers by two or three points toward Mr. Romney in order to make it equivalent to a likely-voter poll.

(More here.)