Kate Dailey provides numbers on it:
[E]xperts predict that at least 35% of those who vote in this election will cast an early vote this year, up from 30% in 2008. And in crucial swing states like Florida, Colorado, Iowa and Ohio, that number might be much higher. "Colorado is one where 85% of the votes are going to be cast prior to election day," says Michael McDonald, an associate professor of government and politics at George Mason University. "Florida is going to be close to two-thirds. In Ohio, they're on pace for 40-45% early voting."
RNC Political Director Rick Wiley claimed this week that "across the eight states, Democrats are underperforming their share of 2008 AB/EV votes cast by a net 5.85 percentage points, while Republicans are over-performing their share by 2.13 points, yielding a net swing of +7.98 percentage points for Republicans." Jim Geraghty pushes back:
I have two very minor quibbles with the bright portrait he paints. First, in a bunch of swing states, Obama won by considerable margins – 120,000 votes in Nevada; 130,000 votes in Virginia; 150,000 votes in Iowa; 240,000 votes in Florida; 400,000 votes in Wisconsin, 600,000 votes in Pennsylvania. So Romney doesn’t have to do merely better than McCain did in 2008, he has to do considerably better; secondly, the under-funded McCain effort did such a poor job in early voting in these states that it’s not the highest bar to clear.
Ezra Klein is more blunt:
Absolutely everything I’ve heard suggests the Obama campaign is meeting and exceeding its early voting targets. You can see some on-the-ground evidence of this from Jon Ralston’s look at early voting in Nevada, which is showing huge numbers for the Democrats, and the Time poll of Ohio, which showed a huge lead for Democrats among early voters. Democrats also appear to lead in early voting in North Carolina.