Vice President Kamala Harris tried to boost her soft support among union voters on Friday, speaking at the International Brotherhood of Electrical Workers office in the battleground state of Wisconsin.

Leaders from labor unions have enthusiastically endorsed the Democratic nominee, but her support among the rank-and-file workers, a once traditionally reliable Democratic voting bloc, have been gravitating toward her Republican rival, former President Donald Trump.

At IBEW Local 890, Ms. Harris cast herself as a friend of unions who will fight for their interests and who understands their concerns.

“I proudly stand with labor. I have my whole entire career. I always will,” she told the crowd.

“Every person in America benefits from your work. I tell people everywhere I go: ’Thank a union member,’” Ms. Harris continued. “Thank a union member if you got a five-day workweek; thank a union member for your sick leave; thank you a union member for paid family leave because it is union members that work and put blood, sweat and tears into raising the conditions of the American worker, wherever they work.”

Ms. Harris told the Wisconsin union members that Mr. Trump is no friend of organized labor and will weaken federal worker protections.

“Donald Trump’s track record is a disaster for working people and he is an existential threat to America’s labor movement,” she said, referring to the former president as a “union-buster.”

Polls show Ms. Harris has work to do to bring skeptical union workers back into the Democratic fold. An internal Teamsters poll found that its Michigan members preferred Mr. Trump (61.7%) to Ms. Harris (35.2%), prompting the national union’s leadership not to make an endorsement in this year’s presidential race.

Ms. Harris also failed to gain the support of the International Association of Firefighters.

Despite declining private sector membership in recent decades, union households will likely prove critical to winning this year’s election. Union volunteers have long served as foot soldiers in Democratic organizing and get-out-the-vote efforts.

In the battleground states of Michigan, Nevada and Pennsylvania, roughly 15% of employees belong to a union. The total number of union workers in all three states exceeded the margin of victory in those states in the 2016 and 2020 elections.

In 2020, Joe Biden won 57% of the union vote in swing states and Mr. Trump received 40%. Ms. Harris would need to at least match Mr. Biden’s total to win the election.