Professor Michael P. McDonald of George Mason University, one of the nation’s leading experts on voter registration data, has calculated that since November 2008, the number of registered voters in the states that have registration by party has declined 2.6%. This is not surprising, because elections officials customarily work between elections to purge the lists of deadwood. See his summary here.
McDonald has also calculated that since November 2008, the number of registered Republicans has declined 3.5%, the number of registered Democrats has declined 2.7%, the number of registered independents has declined 1.6%, and the number of voters registered in minor parties has increased 2.4%. McDonald also notes that research has shown that voters who are registered in minor parties are more sophisticated and more politically active than average voters. Thanks to Rick Hasen for the link.