So how did Johnson win? The biggest thing he did was campaign on not being a creature of Washington, a fresh face appealing to an atmosphere of anti-incumbency. As he explained in this ad, he's not a lawyer:
Johnson's other theme was to attack federal spending, strangely turning Feingold's past fiscal conservatism and anti-pork crusades right back at the incumbent in the wake of the federal stimulus and other Obama administration programs:
Johnson stuck to his themes -- sometimes to a fault. This became clear after his multiple admissions that he doesn't actually have any detailed policy proposals, only a general conservative philosophy and his manufacturing business background. "When I interview people, I don't expect them to come into that interview and say here are the problems and how to solve them," he famously said.
This flap cost him the endorsement of at least one newspaper, the Green Bay Press Gazette, which endorsed Feingold for the first time ever. But ultimately it wasn't enough to cost Johnson the race.
In addition, one thing that definitely tripped up Feingold this year was that a lot of Democratic-leaning Wisconsinites simply didn't vote.
As a recent Public Policy Polling (D) survey found:
Wisconsin has one of the largest enthusiasm gaps of any state in the country. Although it appears Democrats will have turnout issues pretty much everywhere the problem is unusually severe in Wisconsin. Those saying they're likely to vote this fall report having voted for Barack Obama by only 3 points in 2008. He actually won the state by 14 points.
A Marist poll found Feingold trailing by 52%-45% among likely voters. But among the wider pool of registered voters it was a much closer race, with Johnson only ahead by 44%-42% -- a deficit that would have been much more feasible to overcome.
Feingold's main legacy is the McCain-Feingold campaign finance reform law. But even here, the Washington Post noted this week, that law has been steadily worn down by the Roberts Court in a series of Supreme Court rulings, most notably the Citizens United decision that reopened the floodgates of corporate money in politics.
In addition, Feingold will be remembered as the only vote in the Senate against the Patriot Act, expanding the surveillance powers of the federal government, which was hastily passed in the wake of the 9/11 attacks in 2001. At Feingold events in 2004, he frequently told constituents that he was the only Senator to vote against the Patriot Act -- because he was the only Senator to read the Patriot Act.
Late Update: Watch Feingold's concession speech here.