From left-leaning Third Way, an analysis suggesting that Mitt Romney’s consistent lead among independents might prove more crucial than anticipated (emphasis mine):
Our final analysis of voter registration changes since 2008 finds that Democrats have shed 372,827 voters and Republicans have gained 158,037 voters. Of note is a recent surge in Democratic registration in these swing states—an increase of over 400,000 in the past 3 months. But the real change is with Independents, who have increased by nearly 1 million in the 8 presidential battleground states that keep voter registration data by party.*
Democrats do have an edge in early voting:
Only 5 presidential battlegrounds with partisan voter registration have released early/absentee voting figures by party identification. On average in those 5 states,Democrats maintain a combined edge, although individually the party has an edge in only 4 of 5 states. As of November 1, 2012, 43.6% of the early votes have been cast by Democrats, 36.8% by Republicans, and 19.6% by Independents.‡ Since early voting field operations tend to target partisans and not Independent voters, it is expected that the number of Independents voting early will be considerably less than Democrats and Republicans.
However, the GOP has placed more emphasis on Election Day voting than early voting, and furthermore, plenty of Democrats voting early aren’t sporadic voters, but people who would have gone to the polls on Election Day anyway.