I'm going to spoil the big surprise up front: Barack Obama is more liberal than Dennis Kucinich.
Now the context. National Journal has put out a series of lists in which they rate every lawmaker in the House and Senate on how they voted in 2006. (There's a link on the Mother Jones News and Politics page, your home for 2008 presidential coverage and general Washington news.) You can see the most liberal and most conservative members of Congress. You can see where Lieberman stands (not the most conservative Dem). And perhaps most interestingly, you can see where the presidential candidates fall.
The New York Times political blog dug a little deeper and found lifetime ratings. The results?
On a scale of one to 100, with 100 being the most liberal, here are the Dems:
Senator Barack Obama: 84.3
Representative Dennis Kucinich: 79.4
Senator Christopher J. Dodd: 79.2
Senator Hillary Rodham Clinton: 78.8
Senator Joseph R. Biden Jr.: 76.8
On a scale of one to 100, with 100 being the most conservative, here are the Republicans:
Representative Duncan Hunter: 82.5
Senator Sam Brownback: 81
Representative Tom Tancredo: 75.9
Senator John McCain: 71.8
Senator Chuck Hagel: 71.5
Representative Ron Paul: 51.7
Due to lack of votes in Congress, certain contenders for the nominations are left off.