This is pretty good:

http://votesmart.org/voteeasy/

It's *really* hard to nail them down on issue positions during the primary season, though, precisely because of the negative attacks and walk-backs of previous statements. Peoople have basically given up on expecting candidates to run honest campaigns during the primaries -- they know their candidate is going to have to soften their message to reach independents in the general -- so there's almost no interest from voters or the media in nailing them down on issues. It's kind of sad.