Teachers are the new heroes of liberal America. Thousands have walked out of classrooms and packed the state capitols of Oklahoma and Kentucky, weeks after West Virginia teachers launched a state-wide walkout over low pay and a broken insurance system and won most of their demands. The liberal media has embraced the protests, which at first seems natural. Public schools are generally underfunded, after all: Every state in the country faced a teacher shortage at the start of the 2017-2018 school year. And with the loathed Betsy DeVos helming the Department of Education, the future of public education seems to tremble on a precipice.

But liberals haven’t always defended teachers with such fervor, as the historian Corey Robin recently noted at his website. In 2012, many greeted striking Chicago teachers with cynicism and even open hostility. So what has changed?

The teachers’ protests are an important lesson in the ways Donald Trump’s presidency has papered over historical divisions on the left. The Trump era began with massive protests, which had the effect of turning public demonstrations—normally a tactic for activists and workers and unions—into a more mainstream form of civic engagement. It has also coincided with a historic weakening of union power, which has pushed teachers and their representatives away from the negotiating table (with its connotations of backroom dealing) and into the clear open air of the streets. And it has turned protests against the arrayed powers of the dark side into a cause to celebrate—even when a Democratic administration largely ignored similar protests.

Robin provides a few representative examples of the way liberals used to respond to teachers’ strikes. Former Time education columnist Andrew Rotherham: “Part of this strike, it’s pretty clear, is that the union needed to have some theater for its members, let them blow off some steam, and that’s increasingly obvious.” Robin added, “I got into a Twitter spat with ABC News’s Terry Moran, who tweeted, ‘I wonder if the Chicago teachers realize how much damage they are doing to their profession—and to so many children and their families.’”

The fear that such strikes would defeat their own purposes and damage teachers’ allies was not limited to the media. It struck the Democratic Party, too. When Chicago teachers went on strike in 2012, demanding fair evaluations, protections for their health benefits, and basic air conditioning in their classrooms, among other things, observers in the press and in the Democratic Party worried about its implications for Barack Obama’s re-election campaign. Days before Chicago teachers even walked out, the Democratic National Committee signed off on a screening of pro-charter school drama Won’t Back Down at the 2012 convention. “Any time we’re fighting among ourselves, it’s never helpful. Any time you waste money and time fighting each other, it’s money we’re not spending fighting our real opponents,” one Democratic strategist told The New York Times.