There is an ancient saying that is translated into modern English as "the enemy of my enemy is my friend."
This philosophy is likely responsible for the popularity of Ron Johnson among the state's Tea Party voters.
Johnson has proven to be a liar and a hypocrite on the subject of entrepreneurship (he earned his money the old-fashioned way -- he married it), government-sponsored health care (he took his daughter to a state-university-owned hospital in Minnesota for her surgery and has encouraged his employees to enroll in BadgerCare) and Randian economics (he sought government-subsidized bonds and railroad tracks for his company and stimulus funds for the Oshkosh Opera House, on whose board he serves). He has flip-flopped on the subject of gun control (originally stating that gun owners should be licensed like drivers, until he learned that position was anathema to his base). He has approved outright lies about Russ Feingold's record.
Feingold, on the other hand, is one of the Senate's staunchest supporters of 2nd Amendment rights (he even filed an amicus brief on behalf of McDonald in the recent McDonald v. Chicago case). He famously voted against the USA Patriot Act. He often votes against his party, though as a liberal Democrat, he is in agreement with (and therefore votes for) most of their legislative agenda.
It is those liberal leanings which have earned the ire of the Tea Party. If Feingold's only opponent on the ballot were Beelzebub, the Prince of Darkness would have their vote.