REDFORD CHARTER TOWNSHIP, Mich. — Vice President Kamala Harris denounced Donald Trump as a “union buster” as she campaigned Friday in the labor stronghold of Michigan, stepping up her political battle with the former president for the support of working-class voters who could tip the scales in this year’s election.

Harris’ remarks followed U.S. dockworkers suspending their strike in hopes of reaching a new contract, sparing the country a damaging episode of labor unrest that could have rattled the economy. A tentative agreement was reached to raise salaries, although other issues need to be resolved.

Visiting a firehouse in Redford Charter Township, outside Detroit, the vice president shouted out to “labor brothers and sisters” that the tentative dockworker agreement demonstrated the power of collective bargaining.

“We know, when union wages go up, all wages go up,” Harris told a small crowd featuring representatives from different unions. “And the bottom line is, when unions are strong, America is strong.”

She also directly went after her opponent, saying Trump “has been a union-buster his entire career” who would launch a “full-on attack” against organized labor.

Trump is “making the same empty promises to the people of Michigan that he did before,” Harris said, “hoping you will forget how he let you down.”

Harris was set to speak later in Flint and her campaign said she’d use those remarks to criticize Trump and his running mate, JD Vance, on auto jobs. She planned to point directly to Vance, who while campaigning Wednesday in Michigan refused to commit to continue federal support going to the GM plant in Lansing.

Unions have long been a bedrock of support for Democrats, but Harris has failed to secure some key endorsements. The International Association of Fire Fighters announced this week that it wouldn’t back a candidate this year, following a similar announcement from the Teamsters. Both unions endorsed Joe Biden four years ago.

Still, Harris was introduced and endorsed at the firehouse by Matthew Sahr, president of Michigan firefighters. Similarly, some Teamsters groups have publicly backed the vice president even as their national union has opted not to make formal endorsements.

There were nonetheless warning signs for Harris.

Justin Pomerville, the business manager at UA Local 85 in Michigan, said 70% of his members’ work hours are tied to the CHIPS and Science Act, which the Biden administration championed and pumped billions of dollars into semiconductor manufacturing.

The workers lay complex networks of pipes that carry exhaust, water and chemicals through high-tech facilities. However, Pomerville said some members aren’t aware of the connection between their jobs and the legislation.

“Unless someone tells them they’re working because of that, they don’t know,” he said.

It’s not a total cold shoulder for Harris. She won quick endorsements from national teachers unions, the building trades, the AFL-CIO and the United Auto Workers after replacing Biden at the top of the Democratic ticket.

But the divide within the labor community is a reminder of shifting loyalties in American politics. Democrats have increased their support among white-collar professionals while Republicans try to make inroads among voters who didn’t attend college.

During a rally Thursday in Saginaw, Michigan, Trump claimed that Republicans are now “the party of the American worker,” glossing over his anti-union record as president. In a reference to labor unrest at the country’s ports, he insisted that “under my leadership Americans won’t have to go on strike for a better wage or a better life.”