15:9

That's the ratio of the number of people who voted for Democrats compared to the number of people who voted for Republicans on Super Tuesday. Data analysis by Jackson Dykman, links courtesy of Karen Tumulty, both of TIME.

Democrats:

Clinton: 7,378,275 (corrected from previous version)
Obama: 7,319,607 (corrected from previous version)
Edwards: 409,723
Uncommited: 26,268
Biden: 2,863
Richardson: 1,164
Total of All Dem Primary Votes: 15,137,900

Republicans:

McCain: 3,613,133
Romney: 2,964,010
Huckabee: 1,796,474
Paul: 397,229
Giuliani: 199,490
Uncommitted: 10,528
Thompson: 16,044
Total of all Republican Primary Votes: 8,996,908

UPDATE: commenter anon (here & here) at Tumulty's blog did some more digging and found some more evidence that the Dems are more enthusiastic than the Reps this year.

To recap
R's, 2000: 9,526,080
R's, 2008: 9,531,712

D's, 2000: 7,731,127
D's, 2008: 15,321,181