Russ Feingold is in. If Democrats want to retake the Senate, he better win.

The former Wisconsin senator and liberal icon announced Thursday that he will seek a long-expected rematch with first-term Republican Sen. Ron Johnson, setting up one of the most consequential Senate races of 2016. Feingold lost to the tea-party-backed Johnson in 2010, which was among the most unexpected and devastating defeats Democrats suffered in that year's wave election.

To Democrats, it was also a fluke. And they're hell-bent on proving it because in Wisconsin, the stakes are much higher than a single Senate race: Without a Feingold victory, the party has very little chance of winning control of the Senate. A Feingold loss would likely indicate a very strong year for Republicans nationally. And in such a case, it's difficult to imagine how Democrats could knock off enough better-entrenched incumbents in redder states to overcome such a setback in Wisconsin.

Democrats need to win at least a net of five Senate seats next year to guarantee a majority. (The number shrinks to four if they win the presidency.) But even on a map laden with Republican incumbents defending blue-state seats, Democrats have few better opportunities than Wisconsin, where Johnson is widely considered, even by Republican operatives, to be the GOP's most vulnerable incumbent. (Illinois Sen. Mark Kirk also is in the running.)

Unlike fellow blue-state Republicans up for reelection next year, like Pat Toomey and Kelly Ayotte, Johnson hasn't worked as diligently or effectively to moderate his image or stuff his campaign's bank account. And while Wisconsin has trended red in recent midterm elections, Democratic presidential candidates have won the last seven contests there, including Barack Obama's six-point victory in 2012.

Feingold might even be the rare challenger to enter the race as a favorite. A Marquette University Law School poll in April found Feingold with a massive early lead over Johnson, 54 percent to 38 percent—a result that speaks not only to Feingold's personal popularity four years after leaving office (he's still better-known than the current senator) but also to Johnson's low marks. In an era where even most well-positioned incumbents rarely crack 50 percent in early head-to-head matchups with prospective opponents, the survey's results were eye-popping.

They're encouraging signs for the former senator, but they also put pressure on his campaign. Without Wisconsin, Democrats would need to run the table in less-inviting states such as New Hampshire, Pennsylvania, and Florida or pick up an unexpected win in more conservative-leaning locales like North Carolina or Indiana. (Meanwhile, the party also has to defend seats against potentially stiff challenges in Colorado and Nevada.) That's not impossible, but it's a much more difficult path.

Feingold, of course, will face a litany of challenges against Johnson. While Democrats think his loss in 2010 was the product of a poorly run campaign and wretched political environment, Republicans see a burnout politician on whom voters soured when presented with the fresh-faced Johnson.

Six years later, even with Johnson as the incumbent, GOP strategists think they can still frame the race as one between a political outsider and a career politician. Helping their cause, they say, is the belief that Feingold, a crusader for campaign finance reform, will green-light the help of political committees and super PACs—the kind of outside spending he once made a career out of opposing and eschewed in 2010.

"After decades in politics, Feingold's ego still can't grasp that he was soundly defeated by Oshkosh job creator Ron Johnson in 2010," said Andrea Bozek, spokeswoman for the National Republican Senatorial Committee. "Wisconsin families rejected Feingold's broken promises and his liberal record once and they are going to do it again. Wisconsin voters know a desperate career politician when they see one and that is why they will re-elect their independent leader, Ron Johnson."

Officials at the NRSC, who have spent months preparing for Feingold's return, also point out that the last defeated senator to win a Senate comeback attempt was Washington Republican Slade Gorton, in 1988. And the possibility of a home-state presidential candidate, Gov. Scott Walker, topping the 2016 ticket is another wild card.

With support from Washington and Wisconsin Democrats, Feingold is all but assured of a ticket to the general election. He remains popular with the state's liberal base and his interest in returning effectively froze the field in Wisconsin for months, preventing possible contenders such as Rep. Ron Kind from even thinking about running. Minutes after Feingold officially entered the race, the Democratic Senatorial Campaign Committee announced that he had its support. It's yet another sign of how consequential Wisconsin is for the committee in 2016.

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