As of November 2012, 35 percent of registered voters were Democrats; since there were 146,311,000 Americans registered to vote in that year, there were approximately 51,208,850 registered Democratic Party members. As of 2015, 48 percent of the general public identifies as or leans Democrat, though this figure includes non-registered citizens.
The exact number of registered voters in each party, at a nation-wide level, is not tracked by any official government entity. The United States does not maintain official records on party affiliation by individuals, and the Democratic Party by and large uses a private company called NGP VAN to track registration statistics. NGP VAN reported 2,337,484 voters were registered Democrat or leaned progressive during the 2014 mid-term election cycle. However, this does not account for voters who registered earlier.
While the Pew Research Center claims that, as of 2015, 48 percent of the general public leans Democrat, researchers at Gallup found that 29 percent of the public identified as Democrat in their October 2015 polls on party affiliation. In either case, the party has a slight lead over the number of Americans affiliated with or leaning towards the Republican Party.