Tad Devine was in a car driving from Rhode Island to Washington when he told 125 reporters that Hillary Clinton was going to lose the general election. It was 2016, Bernie Sanders had just won New Hampshire in a landslide, and Devine—Sanders’s chief strategist, a 30-year Democratic operative who had worked for Al Gore and John Kerry—was making the case that the party was about to nominate the wrong person.

Look at Clinton’s weakness in open primaries, where independents could cross over and vote, Devine said. Those same voters would hand the election to Donald Trump in November.

“I said on that phone call that I thought Hillary Clinton was a weak candidate in the general election,” he told Fortune. “And then I was lambasted for the next two weeks. Called a misogynist and everything else because they said I was calling a woman a weak candidate.”

Trump won Wisconsin, Michigan, and New Hampshire. Every state Devine had flagged.

A decade later, he’s written a book about it, called How the Democrats Screwed Bernie, out July 7. But his argument isn’t really about Bernie Sanders at all. It’s about power, who holds it inside the Democratic Party, how they protect it, and what it costs the party when voters are cut out of the equation.