I hope not he cost us the seat in 2010 because of his the poor campaign he ran.
Russ Feingold is not a household name that most Americans know. Despite that, I recall my mother bringing multiple articles to my attention about how he was running an incredible campaign and was traveling all over the state because he cared deeply about winning a 4th term. He did nothing poorly (except perhraps not going negative?). This wasn't exactly a Udall situation. 2010 was just a disaster and there was a huge enthusiasm gap - mostly national but also with Walker excitement I believe. It would have been hard to do better than Feingold did.
Feingold would be heavily favored in 2016 even with Walker on the GOP ticket.
The problem with Feingold is that he refused to accept outside help. That's what happens when you unilaterally disarm. If he decides to change his mind on that issue, then I'd fully support him. If he intends to do the same thing, then hopefully Kind runs instead.