In the summer of 1894, a Democratic Congress easily passed and a Democratic president promptly signed a bill making Labor Day a holiday for federal workers. Samuel Gompers, the era’s most prominent union leader, welcomed the move, though he also called it merely a “slight concession” to workers at a time of rising unemployment and hunger.

Then, just a week after inking the measure, President Grover Cleveland took a step that had a far greater impact on him and his party. He dispatched 2,000 federal troops to Chicago to break a strike by railroad workers that was crippling the economy.

The unprecedented act antagonized many of the wage earners who had helped elect Cleveland two years earlier. In midterm elections that fall, Democrats lost more than 100 seats in the House and two in the Senate. A day after the debacle, Gompers wrote to Cleveland: “Without much concert of effort by organized labor the people have answered at the polls your assumption of an unconstitutional and unwarrantable use of the military power to crush labor.”

As a fledgling union revival spreads across the country, Democrats would do well to recall the lesson of those events: They need unions to win. But unions also need Democrats, and now is the time for the party to step up to promote the kind of organizing that will benefit both sides in the future.

Democrats today are voicing rhetorical support for organized labor as strongly as they have at any time in the party’s history. “We should encourage unions,” President Biden remarked in June. “I’m not just saying that to be pro-union. I’m saying it because I’m pro-American.” Biden also met with Christian Smalls, the young Black leader of a drive to sign up workers at Amazon. (Amazon founder Jeff Bezos owns The Post.)

But sympathetic rhetoric is not enough. The labor revival that Smalls and his fellow activists are stoking remains modest. It has found success mainly among wage earners at firms such as Starbucks and REI, who tend to be young college graduates. The labor upsurge has yet to reach those Whites and Hispanics who have spent little or no time in college — sizable groups that have been shifting toward the GOP. Where large numbers of less educated Hispanics do belong to unions, in cities such as Los Angeles and New York, most still vote solidly Democratic.

The now bright-red state of West Virginia offers an object lesson in union power. From the 1930s through the 1990s, White workers there faithfully elected Democrats. Driving the party’s long sway in the state was its bond with the United Mine Workers, which taught its members that Democrats stood for higher wages and protection of the right to organize, and that Democrats were helping fund the free union-run health-care system. Then mechanization and decline of the coal market sliced the UMW down to half its former size. West Virginia became one of the poorest states in the country. Without the union, working-class residents had nowhere they could complain about their lack of economic and medical security — at least in a way that benefited Democrats.