Apart from his blinders on campaign finance, Feingold was a very strong civil libertarian, and the one Democrat I proudly voted for this election.
My regret for his loss this cycle will be second only to that of Prop 19.
Marc Ambinder 
attempts to explain the loss: 
Start from the sociological point: beer. Milwaukee. The Tavern  League is a huge lobbying force. The Democratic base in the state is  compromised of what Ron Brownstein calls "beer track Democrats": blue  collar voters, ethnic whites who tolerate government when it helps but  recoil from it when it seems intrusive and wasteful. They tend to be  older. Sporadic and infrequent Democratic voters in the state come from  the university towns like Madison. Wisconsin has had an unusually high  rate of young voter participation, a trend that dates back to the  mid-90s. People forget that Barack Obama won the state in the primaries,  and that Democrats since Michael Dukakis have kept it in the blue  column in presidential races. It's hard to argue that voters in  Wisconsin are Democrats simply by habit. But at the same time, Democrats  took back the state legislature only last year, and Feingold has never  been reluctant to admit that he takes positions that are somewhat out of  sync with his state. But culturally, he's one of them. Balanced  budgets. Gun rights. And a beer drinker. 
Younger  college-town Democrats and older habitual Democratic voters are  clashing this year on entitlement spending; younger Democrats see the  health care reform law and the stimulus package as down payments on  their future. Older Democrats see it as a waste of money. This trend  plays itself out 
culturally, too.
Beer-track  Democrats tend to be the toughest to turn out this cycle. White men  without college degrees have grasped onto economic libertarianism as a  way out of the fiscal mess. And younger Democrats simply aren't turning  out. 
Feingold has done everything he can to  remedy this. Unlike many Democrats, he's run as an unabashed champion of  health care, the stimulus, energy reform, and progressive cultural  advancement.
[..] Feingold is unafraid to be the avatar of an  argument that Wisconsin residents are just going to reject this cycle.  Health care isn't popular. The stimulus is considered wasteful. The  economy is tough and jobs aren't coming back. The profile of the  electorate is much more conservative. Wisconsin has been run mostly by  Democrats for eight years. It's not in very good shape. Feingold  represents all of that.
True, unfortunately.
It didn't have to be this way. He would have had a fair shot had he de-emphasized the leftism.
I hope his political career isn't over.