Those rankings are suspicious. I thought John Kerry was the most liberal senator!
I think the rankings are directly proportional to status as Democratic nominee.
Insert random figures slightly below said Democratic nominee. Remember 2004's "More liberal than Ted Kennedy?" I think I heard Obama being accused of being more liberal than Bernie Sanders in a similar fashion already.
I mean, a good portion of what those rankings measure is how often you vote with your party. Candidates in the middle of a primary aren't about to side with the opposing party on any issue. Note how John McCain went from voting with Bush ~70% of the time to 95% in 2007. Biden is rated much higher because he was also running for president in 2007 too.
Anyway, yeah, 538 has it tied. Cool.
