*Update* Him being the most liberal is based on his 2007 voting record which contained 99 votes. When compared to the others he came out number 1. Clinton was 16th and Biden was 3. He is rated 0% by the National right to Life, 100% rating with the NARAL and he is rated 100% by the Citizens for Tax Justice, indicating he supports progressive taxation. He also has a history of gun control amoung various and numerous other liberal stances.
(OM)He leans a bit left even for people in San Francisco. I think his cabinet might make the 9th Circuit look moderate. He has enough of a voting record to have made his liberal tracks! With him adding Biden to the ticket(3rd most liberal Senator in the nation, with Obama being number one.) I can't see how any moderates would be willing to swing that far to Left field.