What happened to Russ Feingold?

[CatoMinor]

Was it Trump coat tails or did Feingold do something that screwed himself over?

[Mr. Smith]

While Trump coattails obviously had the most to do with it, I think his cynical and seemingly hypocritical acceptance of some donations undermined him a bit too when in 2010 he barely lost despite lacking money by a longshot.

[Xing]

Democrats in general just took Wisconsin for granted.

[Maxwell]

He got clearly out-hustled by Senator Johnson.

[MT Treasurer]

Johnson ran a strong campaign in a year which was favorable to Republicans; Feingold was seen as an "insider"; WI is not a blue state, etc. This was the first time a Republican won a Senate race in Wisconsin in a presidential year since 1980.

[JMT]

Democrats in general just took Wisconsin for granted.

This. Everyone assumed Feingold and Clinton would win so they didn't try too hard. Johnson ran a good campaign but no one took him seriously till it was too late.

[Cynthia]

He took it for granted, ran a lazy campaign, lost to a good campaigner with lots of fund.

[sg0508]

Johnson ran a strong campaign in a year which was favorable to Republicans; Feingold was seen as an "insider"; WI is not a blue state, etc. This was the first time a Republican won a Senate race in Wisconsin in a presidential year since 1980.

This. The insider label hurt him bad.  He probably also watched the polls too closely, which we knew were wrong. 

[DavidB.]

The problem lies more with Hillary's campaign than with Feingold here. Sure, Johnson overperformed Trump, but if Hillary's campaign wouldn't have neglected WI and the Midwest, Feingold could have won. So yeah, this:

Democrats in general just took Wisconsin for granted.