The country is composed of about 30% Democrats, 30% Republicans and 40% of other or no party.
Judging by the official results of the past 4 POTUS elections, I think it's probably close to about a 50/50 split between conservatives and liberals in the United States.
Here's what I mean:
Popular votes
2012 POTUS election
Democrat votes: 65,446,032
Republican votes: 60,589,084
Total votes: 128,556,837
65,446,032 + 60,589,084 = 126,035,116
126,035,116 / 128,556,837 = 0.98038438826 = 98.038438826% (total votes)
65,446,032 - 60,589,084 = 4856948
4856948 / 126,035,116 = 0.03853646629 = 3.853646629%
So about 98% of total votes were either for the Democrat party or the Republican party, and there's only about a 4% difference between the votes of the Democrats vs. Republicans.
2008 POTUS election
Democrat votes: 69,297,997
Republican votes: 59,597,520
Total votes: 131,032,799
69,297,997 + 59,597,520 = 128,895,517
128895517 / 131032799 = 0.98368895409 = 98.368895409%
So about 98% of total votes were either for the Democrat party or the Republican party.
2004 POTUS election
Democrat votes: 59,027,478
Republican votes: 62,039,073
Total votes: 122,284,939
59,027,478 + 62,039,073 = 121,066,551
121,066,551 / 122,284,939 = 0.99003648356 = 99.003648356%
So about 99% of total votes were either for the Democrat party or the Republican party.
2000 POTUS election
Democrat votes: 50,996,582
Republican votes: 50,456,062
Total votes: 105,363,298
50,996,582 + 50,456,062 = 101,452,644
101,452,644 / 105,363,298 = 0.96288409651 = 96.288409651%
So about 96% of total votes were either for the Democrat party or the Republican party.
Source for raw data:
http://www.archives.gov/federal-register/electoral-college/historical.html