There are 31 states and the District of Columbia where voters must register their party preference. Democrats have the most registered voters (39.66%), and they have for quite a while. Until recently, the Republicans had the second most registrants and Independents were third. That has changed.
For the first time, Independents (29.09%) now outnumber Republicans (28.87) slightly.
Democrats have dropped 2.53 points since 2004, and Republicans have dropped 2.92 points since then. Independents have increased by 5.94 points.
Note that there are not enough Democrats or Republican to carry an election. Both need to appeal to Independents to win. Democrats need to realize this -- and not nominate a candidate that scares those Independents (most of whom are moderates).