While I'm guessing your numbers are right and that there will be quite a few Democrats that vote to confirm, why are you singling out Feingold? Nelson is a safe pick as he is probably the most "republican" of the Democrats, but I'm not understanding the Feingold pick. Has he said something I'm not aware of?
("Denny Crane: Gun Control? For Communists. She's a liberal. Can't hunt.")
Wisconsin is becoming a competitive state for Republicans. Feingold is aware of this, which is why he voted for Roberts. He does have the buffer of being the only Dem to vote against the war, it gives him a little leeway with the Dem base in his quest for their Presidential nomination.
Still, Roberts was one vote not likely to change balance. The question now is the calculation he'll make. Senatorial long term viability vs edging a competitive lead to be president. Two different constituencies to balance in this decision.