Sanders was favored in the state, and polls had consistently given him a lead of 2 to 8 points. Wisconsin is heavily white, has a strong network of youth voters, and is historically receptive to progressive politicians — all of which make it favorable territory for Sanders.
But even though the campaign secured a victory, it's doubtful Wisconsin will significantly diminish Hillary Clinton's commanding lead in the Democratic race.
That's because Sanders doesn't need to win states, he desperately needs to win the delegates that determine the Democratic nomination. Because of the proportional delegate system, Sanders may not actually secure that many more delegates than Clinton after tonight.
Coming into Wisconsin, Clinton led Sanders by more than 220 delegates. The Milwaukee Journal Sentinel found Sanders could win by a big margin and still only gain a handful of delegates compared to Clinton — which obviously won't be enough to do the trick.
How Sanders's win tonight could make a difference
The best news from tonight for Sanders's supporters is that winning Wisconsin has a chance of improving his standing going into the bigger upcoming contests in Pennsylvania and New York, where many more delegates are at play.
Silly as it might sound, political scientists do have evidence that candidates' chances can rise and fall based on media narratives about their perceived "momentum." Sanders's win in Wisconsin seems likely to at least keep that narrative alive.
"We do have evidence that there is such a thing as momentum. Altering perceptions of electability among voters can be really persuasive," said Adam Seth Levine, a political scientist at Cornell University.
Sanders himself appeared to allude to this with his last-minute pitch to voters. "If we win here, we are going to have a bounce going into New York state, where I think we can win," Sanders said in Wisconsin on Monday at a local union hall, according to ABC News. "We win in New York state, we are on our way to the White House."
There are more than a few problems with that narrative: Clinton is leading by a lot in most polls of New York, and Sanders would have to expand his base in profoundly new ways to win that state.
There's also the question of several other states on the map where he'd have to dramatically reverse his fortune — Pennsylvania, Maryland, Connecticut — where Clinton has looked strong.
But for tonight, at least, the Sanders campaign kept hope alive — thin though it might still be.