From Gallup:
An average of 47% of Americans identified as Democrats or said they were independents who leaned Democratic in 2012, compared with 42% who identified as or leaned Republican. That re-establishes a Democratic edge in party affiliation after the two parties were essentially tied in 2010 and 2011. . . .
The new Democratic advantage is mostly due to an increased proportion of Democratic-leaning independents and a decreased proportion of Republican-leaning independents. Thus, the movement comes almost exclusively among Americans with weaker attachments to the political parties.
In recent history, Republicans have almost always lagged behind Democrats in numbers of voters registered. That hasn’t necessarily been debilitating because of the number of independents who lean right, so it’s not good news that that proportion is decreasing right now.