When 1,700 members of United Electrical, Radio & Machine Workers of America Locals 506 and 618 struck at the sprawling Wabtec locomotive plant in Erie, Pennsylvania, Tuesday morning, they got an immediate show of solidarity from one of the most prominent political figures in the United States.

“Americans are sick and tired of corporate America and their wealthy CEOs ripping off working families,” announced Vermont Senator Bernie Sanders, in a tweet dispatched shortly after the picket line was established. “I’m proud to stand with the locomotive manufacturing workers of @ueunion Local 506 and 618 in their fight against GE/Wabtec to maintain decent wages and working conditions.”

That’s the right response from a contender for the Democratic nomination for the presidency. Democrats have to stand in outspoken solidarity with workers, especially when their unions are struggling to preserve manufacturing jobs and maintain fair wages in historic urban and industrial centers such as Erie.

Fights over the direction of American manufacturing, especially in the transportation sector, are not romantic struggles to preserve old or dying industries. Locomotive production is booming internationally because of the role that rail transportation is expected to play in a new and more environmentally sustainable economy—and because new technologies are opening up dramatic new possibilities. The question is not whether there will be locomotive production, but whether new generations of workers will get a fair share of the future.

The strike in Erie pits a union with deep roots in Western Pennsylvania and American manufacturing against a powerful multinational corporation—Webtec (Westinghouse Air Brake Technologies Corporation)—that, after taking charge this week of a former GE Transportation manufacturing facility, has refused to maintain existing protections for workers. “We are extremely disappointed that the company could not see its way to agree to continue the terms and conditions that we have worked under for decades. Their refusal leaves us with no choice but to go out on strike to protect our members’ and our children’s future,” says UE Local 506 president Scott Slawson.

According to UE: “Wabtec’s terms and conditions, which they imposed when they took over the plant on Monday, include the introduction of mandatory overtime and arbitrary schedules, wage reductions of up to 38 percent for recalled and newly-hired workers, and the right to use temporary workers for up to 20 percent of the work in the plant.”

That’s provoked what the union describes as the “first major US manufacturing strike of the Trump era.”

Sanders has been in the thick of this fight. Last week, he wrote Wabtec CEO Raymond Betler a letter that called out the new boss for trying to squeeze concessions out of workers. “Let’s be clear,” noted Sanders. “Wabtec is not a poor company. It is not going broke. Through the first three quarters of last year, Wabtec made a $256 million profit and had enough money to give you a $3.5 million compensation package.”

“Corporate executives must not use the merger between GE and Wabtec to hurt workers,” wrote the senator, who argued that “the Wabtec/GE merger should not be used to take away the hard-fought gains UE has achieved over the past several decades.”

Sanders promised to “provide my full support and solidarity to the workers at this plant to ensure that they achieve a fair and equitable collective bargaining agreement.” And he has done just that, using his considerable social-media presence and public appearances (including a CNN Town Hall event Monday night) to focus attention on what he has described as a struggle that has meaning for “working Americans everywhere.”

Democrats once went out of their way to align with labor, following the lead of Franklin Roosevelt, who declared two years into his presidency that “It is now beyond partisan controversy that it is a fundamental individual right of a worker to associate himself with other workers and to bargain collectively with his employer.” As the years passed, however, the party’s presidential contenders grew more cautious about throwing in aggressively and consistently with unions as had FDR and the Democrats of the past. Jesse Jackson marched with workers during his Rainbow Coalition campaigns of the 1980s, and candidates such as Jerry Brown and Dennis Kucinich made shows of solidarity during their bids in the 1990s and 2000s. But too many prominent Democrats tried over the years to steer clear of industrial conflicts and fights over labor rights—and more than a few of them broke with labor on issues such as trade policy. Even when prominent Democrats did take the side of labor, they often did so in the stilted language of politicians who sought to avoid offense to potential campaign donors.

What distinguishes Sanders is a determination to steer into the struggle. He has a history of joining picket lines and did so during his 2016 bid for the Democratic nomination. He’s actually been to Erie and has appeared with UE union officials and members.

That’s an important detail, since Trump assumed the presidency with an Electoral College win based on narrow victories in three historically industrial states with track records of backing Democrats in presidential races: Pennsylvania, Michigan and Wisconsin. Erie played a role in tipping Pennsylvania toward Trump, as The New York Times reported after the 2016 election. “Election night maps of Pennsylvania for decades included a bright blue crescent in the upper left corner. It was Erie County, a Democratic stronghold with an industrial economy that President Obama won by 16 percentage points in 2012,” the report noted. “But Mr. Trump flipped the county, winning by two points as he carried Pennsylvania, one of the Rust Belt dominoes whose white working-class voters came out in droves for him.”

The fight for states such as Pennsylvania in 2020 will be about more than particular union fights in particular cities. But labor solidarity has to be a part of the equation.

The example that Sanders is setting is vital, not just for his 2020 campaign but for all the Democrats who are contending for the nomination. Union struggles are essential, not just for Democrats but for the fight for economic justice in a country where the gaps between CEO pay and worker salaries, and between rich and poor, has reached epic proportions. As Sanders says, “We will not have a decent standard of living for our people and the political strength that we need unless we build and grow the trade union movement in this country.”