Listening to talk about the upcoming presidential primaries, I was led to wonder what percentage of Americans are registered in each of the main parties and in neither party. I don’t have an aggregate figure, but here’s the breakout by state, from Wikipedia. I’ve bolded the figures in the final column where the state’s “Independent” voters outnumber R’s or D’s (or both).

http://en.wikipedia.org/wiki/Political_party_strength_in_U.S._states
State 2008 Presidential Election Governor Senior U.S. Senator Junior U.S. Senator U.S. House Delegation Democrats Republicans Independent.
Alabama Rep Rep Rep Rep Rep 6-1 34% 48% 18%
Alaska Rep Rep Rep Dem Rep 1-0 15 26 59
Arizona Rep Rep Rep Rep Rep 5-3 32 36 32
Arkansas Rep Dem Dem Rep Rep 3-1 41 31 28
California Dem Dem Dem Dem Dem 34-19 44 31 25
Colorado Dem Dem Dem Dem Rep 4-3 33 35 32
Connecticut Dem Dem Ind(a) Dem Dem 5-0 37 20 43
Delaware Dem Dem Dem Dem Dem 1-0 47 29 24
Florida Dem Rep Dem Rep Rep 19-6 41 36 23
Georgia Rep Rep Rep Rep Rep 8-5 32 44 24
Hawaii Dem Dem Dem Dem Dem 2-0 40 23 37
Idaho Rep Rep Rep Rep Rep 2-0 22 50 28
Illinois Dem Dem Dem Rep Rep 11-8 46 31 23
Indiana Dem Rep Rep Rep Rep 6-3 32 46 22
Iowa Dem Rep Rep Dem Dem 3-2 34 31 35
Kansas Rep Rep Rep Rep Rep 4-0 27 44 29
Kentucky Rep Dem Rep Rep Rep 4-2 56 37 7
Louisiana Rep Rep Dem Rep Rep 6-1 51 26 23
Maine Dem Rep Rep Rep Dem 2-0 33 28 39
Maryland Dem Dem Dem Dem Dem 6-2 56 27 7
Massachusetts Dem Dem Dem Rep Dem 10-0 37 11 52
Michigan Dem Rep Dem Dem Rep 9-6 40 33 27
Minnesota Dem Dem Dem Dem Split 4-4 46 40 24
Mississippi Rep Rep Rep Rep Rep 3-1 38 47 15
Missouri Rep Dem Dem Rep Rep 6-3 37 39 24
Montana Rep Dem Dem Dem Rep 1-0 32 39 29
Nebraska Rep 4 Rep Dem Rep Rep 3-0 34 48 18
Dem 1(f)
Nevada Dem Rep Dem Rep Rep 2-1 42 37 21
New Hampshire Dem Dem Dem Rep Rep 2-0 29 29 42
New Jersey Dem Rep Dem Dem Dem 7-6 33 20 47
New Mexico Dem Rep Dem Dem Dem 2-1 50 32 18
New York Dem Dem Dem Dem Dem 20-8 49 25 26
North Carolina Dem Dem Rep Dem Dem 7-6 45 32 23
North Dakota Rep Rep Dem Rep Rep 1-0 29 38 33
Ohio Dem Rep Dem Rep Rep 13-5 36 37 27
Oklahoma Rep Rep Rep Rep Rep 4-1 49 40 11
Oregon Dem Dem Dem Dem Dem 4-1 42 32 26
Pennsylvania Dem Rep Dem Rep Rep 12-7 51 37 12
Rhode Island Dem Ind Dem Dem Dem 2-0 38 11 51
South Carolina Rep Rep Rep Rep Rep 5-1 33 44 23
South Dakota Rep Rep Dem Rep Rep 1-0 38 46 16
Tennessee Rep Rep Rep Rep Rep 7-2 34 38 28
Texas Rep Rep Rep Rep Rep 23-9 21 45 34
Utah Rep Rep Rep Rep Rep 2-1 20 56 14
Vermont Dem Dem Dem Ind(a) Dem 1-0 29 27 44
Virginia Dem Rep Dem Dem Rep 8-3 36 39 25
Washington Dem Dem Dem Dem Dem 5-4 39 29 32
West Virginia Rep Dem Dem Dem Rep 2-1 54 29 27
Wisconsin Dem Rep Dem Rep Rep 5-3 38 34 28
Wyoming Rep Rep Rep Rep Rep 1-0 24 16 60

(g) Indicated partisan breakdown numbers are from the registration-by-party figures from that state’s registered voter statistics (late 2010 party registration figures provided whenever possible).