The Rhodes Cook Letter has collected primary election returns for 2010, for 33 states.  This data is in the August 2010 issue.  It shows that in primaries so far this year, approximately 16,160,000 voters have chosen to vote in Republican primaries, but only 13,191,000 voters have chosen to vote in Democratic primaries.  This data is always slightly inexact, because some states don’t release the number of voters who chose any particular party’s primary ballot.  Therefore, the calculation is based on the actual vote cast in each primary, generally for Governor or U.S. Senator or U.S. House-at-large, depending on which office had the highest vote total cast.

The 2010 primary participation so far is a sharp reversal from the 2008 presidential primaries, when 37,000,000 voters voted in Democratic primaries, but 21,000,000 in Republican ones.

The Rhodes Cook Letter is published six times per year.  Every issue consists of election data that is not available anywhere else.  It is written by Rhodes Cook, who is also the author of America Votes, the book published every two years by Congressional Quarterly that has election returns for all federal offices and all gubernatorial elections.  The Rhodes Cook Letter costs $99 per year and can be contacted at PO Box 574, Annandale Va 22003.