Just for the heck of it, I looked up how Clinton and Obama do with American Conservative Union and the Americans for Democratic Action, the two main bellwether groups who rate senators' (and congresspersons') voting records on whether they are conservative or liberal. Again, it's an amazingly dead heat.
On a scale of 1 to 100, with a 100 being perfectly conservative, the American Conservative Union gives a lifetime rating to Obama of 8 and to Clinton of 9.
The Americans for Democratic Action give much more detail. They rate each senator on 20 issues, with 5 points taken off for each time they vote against the agenda. The resulting score is their "Liberal Quotient" or LQ, with 100 being perfectly liberal. You can find a detailed description of what votes they were counting for the last 3 years (the length of Obama's Senate career):
http://www.adaction.org/ADATodayVR2005.pdf
http://www.adaction.org/ADATodayVR2006.pdf
http://www.adaction.org/2007.pdf
And what do you know, Clinton and Obama get exactly the same score each year:
2005: 100 2006: 95 2007: 75
Michael