Ezra Klein has created an easy to understand chart on the weaselhood of Evan Bayh:
I went back in the rankings through the 107th Congress -- which began in 2001 -- to compare Bayh and Nelson's ideological consistency. The numbers on the Y axis represent how conservative of a Democrat each senator was. So #1 would be the most conservative Senate Democrat and #5 the fifth most conservative Senate Democrat and so on. The blue line is Bayh. The red line is Nelson.
... The two really interesting data points, however, are the 109th Congress, which stretched from 2005 to 2007, and the 110th Congress, which ended in January of this year. In the 109th Congress, Bayh's voting pattern suddenly develops an uncharacteristic liberalism. He becomes the 19th most conservative member, with a record more liberal than, among others, Joe Biden. As context, there were also the years when Bayh was preparing for the presidential run that he eventually aborted.