The Gallup survey measures both those who identify as members of a party and independent voters who say they are unaffiliated but admit to regularly favoring one party over the other. Of those surveyed, 31 percent said they were dedicated Democrats and 28 percent said they identified as Republicans, essentially unchanged from 2011.

Independents did make noticeable party shifts over the past year, however. While 18 percent of Americans identified as independents leaning Republican in 2011, only 14 percent did in 2012. Similarly, the 14 percent that said they were Democratic-leaning independents in 2011 increased to 16 percent in 2012. Of all Americans surveyed, 11 percent identified as independents without a political leaning.

"The new Democratic advantage is mostly due to an increased proportion of Democratic-leaning independents and a decreased proportion of Republican-leaning independents," said Gallup's Jeffrey M. Jones in a statement. "Thus, the movement comes almost exclusively among Americans with weaker attachments to the political parties."

The number of Americans identifying as politically independent hit 40 percent again in 2012, matching the record high in 2011. 

"That is particularly notable, given that the usual pattern is for the percentage of Americans identifying as independents to decline in a presidential election year," Jones said. "In each of the last four presidential election years, dating back to 1996, the percentage of independents was lower than in the year prior to the election."