[See update below]
A few years ago I did a study looking at the use of field offices by the Obama and McCain campaigns in 2008. One of the findings was that Obama was absolutely overwhelming McCain in the establishment of field offices. (The Obama folks established over 700 offices nationwide, compared to fewer than 400 for the McCain team.) I had assumed that this difference was the result of Obama having so much more money to spend.
But look at what's happening this year. I went through the campaign websites (here and here) to tally up the number of field offices in the 11 swing states I examined for my paper. Romney has no shortage of money, but Obama's field offices outnumber Romney's in each state.*
It may also be that field offices are a better investment for Democrats than for Republicans. (Notably, my paper found that Obama's 2008 field offices helped him win in a few states, while McCain's didn't help him nearly as much.) Democrats traditionally turn out at lower rates than Republicans; perhaps the field offices can help Democrats mitigate that. Also, field offices may be vital for Democrats in overcoming new voter ID laws by communicating voting requirement information directly to voters.
*I'm not sure what's going on with Indiana. I find it unlikely that Romney's team actually has no offices there at all. This could be a record-keeping problem.
Update on 9/4/12: I'm not sure what happened since last week, but I now can't confirm that either campaign has a presence in Indiana. I've updated the chart above to reflect that.