No one expected Bernie Sanders to win the Michigan Democratic primary Tuesday night by 20,000 votes. Sanders himself did not appear to expect it, convening an impromptu press conference in Florida in front of a nondescript wall, rather than a crowd of supporters, to express his gratitude. Hillary Clinton, after all, held a 21.4-point lead in the RealClearPolitics average of Michigan polls. So why did she lose?

To begin, polls aren’t always accurate and they are not, scientifically speaking, predictive. Still, average polling is the best metric we have to guess how states will vote—a fact that may be cold comfort to the Clinton campaign as it looks ahead to more Rust Belt primaries in Illinois, Ohio, and Wisconsin. In Michigan, Clinton had staked her success on carrying over the same amalgam of liberal talking points plus a specific focus on issues affecting the black community, such as criminal-justice reform and the water crisis in Flint. During Sunday’s debate, Clinton knocked Sanders to the ground by attacking him for his opposition to the 2009 federal bailout, credited with saving the auto industry, the still-beating heart of Michigan’s once great manufacturing base. But while she won Mississippi handily that same night with her focus on civil-rights issues, her message faltered in Michigan where she lost white men by 27 points. At the same time, Sanders made significant inroads with black voters, winning a respectable 31 percent—nowhere near the shellacking he suffered in Alabama, Georgia, and other southern states where Clinton has dominated.

Nothing illustrates Clinton's loss more than the fact that she nearly lost Flint itself. Days earlier, The Washington Post predicted that her strategy of running “like it was a Senate race” would carry her to victory, pointing to her hyper-local focus on the majority African-American city. For weeks, she had campaigned on the Flint’s water crisis, blanketing the city with her surrogates, winning the mayor’s endorsement, and insisting that Sunday’s Democratic debate be held there. But despite her laser focus on the city and calling on Governor Rick Snyder to resign, Clinton eked out a five-point lead in Genesee County.

While Clinton has often criticized Sanders for being a “one issue” candidate, her over-reliance on black support could be her own undoing. Weeks ago, when it became clear that Clinton would lose New Hampshire, her campaign shifted tactics, dismissing the Granite State (and, to some extent, Iowa, where she won by a hair) as overwhelmingly white, arguing that while Sanders may be competitive in less diverse states like Vermont, which he won in a landslide, he couldn’t succeed nationally. Now, that formulation may be coming back to haunt her as the Democratic race moves out of the South, where Clinton steamrolled Sanders, and into whiter, delegate-rich states like New York and Pennsylvania.

Clinton emerged from the fracas Tuesday with more delegates, leading Sanders 87 to 69. But losing Michigan is a worrisome sign for the former secretary of state as her campaign gears up for next week's primary in Ohio—another state where she appears to have a 20-point lead. It will be troubling, too, if Clinton faces a general-election challenge from Donald Trump, who trounced his rivals in the Great Lakes State and remains popular throughout the Rust Belt. If Trump’s and Sanders’s populist appeals continue to hold sway in the northern U.S., Clinton will need to find a way to tap into that same anger in order to keep both at bay. Already, Sanders has narrowed the gap with Clinton in national polling, and head-to-head match-ups with their Republican rivals give the self-described Democratic socialist senator the edge. Perhaps her electability argument is not as rock solid as once thought.