Harris and Biden Will Make a Labor Day Pitch to Union Voters

President Biden and Vice President Kamala Harris will appear together on Monday in Pittsburgh to celebrate labor unions, a crucial mobilizing force for Democrats, at their first joint campaign event since Ms. Harris replaced Mr. Biden at the top of the ticket in July.

Mr. Biden has called himself the most pro-union president in history, and last year he became the first sitting president to visit a picket line.

Now Ms. Harris is hoping to adopt that pro-worker mantle for herself. As vice president, she led a task force examining the ways that the government could help expand union membership. But business leaders generally see her as friendlier to their interests and more flexible on policy than Mr. Biden.

In Pittsburgh, Ms. Harris and Mr. Biden will attend an event at a local union hall alongside Gov. Josh Shapiro of Pennsylvania and Senator Bob Casey, who is up for re-election. The Harris campaign said local and national leaders of major unions including the International Brotherhood of Electrical Workers, the A.F.L.-C.I.O. and the United Steelworkers will attend.

Earlier in the day, Ms. Harris is set to hold an event in Detroit, another union stronghold in a battleground state. She will be joined by Gov. Gretchen Whitmer of Michigan, as well as two of the nation’s most prominent labor leaders, Randi Weingarten of the American Federation of Teachers and Shawn Fain of the United Automobile Workers. Her running mate, Gov. Tim Walz of Minnesota, will appear in Milwaukee.

Although Ms. Harris and Mr. Walz will reach voters in three of the top swing states, the events themselves will be smaller than the rallies Ms. Harris has held in recent weeks that have drawn thousands of attendees. She and Mr. Biden are expected to deliver “informal remarks” rather than full speeches, according to the Harris campaign.

Even as union leaders overwhelmingly throw their support behind Democrats, former President Donald J. Trump maintains the loyalty of many of their members. He is also courting the endorsement of the Teamsters, whose leader, Sean O’Brien, spoke at the Republican National Convention. Winning over the most committed union members is not easy for Republicans, however, as their policies tend to favor corporations.

On Thursday, Senator JD Vance of Ohio, Mr. Trump’s running mate, was heckled at a meeting of the International Association of Fire Fighters after declaring that he was part of “the most pro-worker Republican ticket in history.”

While Ms. Harris is almost certain to receive a warmer welcome on Monday, the war in Gaza continues to alienate some progressives, and unions have been some of the most vocal groups calling for a cease-fire in Gaza and the halting of military aid to Israel. When Ms. Harris addressed a convention of the Service Employees International Union in May, roughly three dozen union members protesting the Gaza war chanted and held signs for nearly her entire speech. S.E.I.U. leadership allowed the protest to go on.

Before dropping out, Mr. Biden had struggled in the polls with union members, along with other core groups of Democratic-leaning voters. In May, 47 percent of union members across six of the top battleground states said they supported Mr. Trump, compared with 42 percent for Mr. Biden, a New York Times/Siena College/Philadelphia Inquirer poll found. Other surveys showed Mr. Biden ahead with union households, but not by wide margins.

Since stepping aside after intense pressure from Democrats, Mr. Biden has shown nothing but warmth for his vice president. During his speech at the party’s national convention in Chicago last month, he said that picking her as his running mate was the finest decision he had made in his career and promised to be her campaign’s “best volunteer.” For her part, Ms. Harris has led chants of “Thank you, Joe” at her rallies and embraced many of his accomplishments.

In an interview with CNN on Thursday — her first since becoming the new face of the Democratic Party — Ms. Harris said that she did not regret defending Mr. Biden against claims that he had declined mentally.

“He is so smart and loyal to the American people,” she said.

Labor Day is traditionally when presidential campaigns move into high gear. Ms. Harris has visited each of the battleground states at least once since July, and Mr. Biden is now set to hit the trail as one of her surrogates.

They are not expected to campaign together frequently however. Ms. Harris is trying to forge her own political identity separate from Mr. Biden, without disrespecting him or his administration’s achievements. Over the next few weeks, Mr. Biden will travel mostly to the key battleground states of Pennsylvania, Wisconsin and Michigan, where he still holds appeal to the white, working-class voters who could help decide the election but who have not shown much enthusiasm for Ms. Harris.

This week could provide a preview of how the Harris campaign plans to deploy Mr. Biden.

After meeting Ms. Harris in Pittsburgh on Monday, the president will travel on Thursday to Wisconsin and on Friday to Michigan to highlight his efforts to finance major infrastructure projects and create manufacturing jobs.

Chris Cameron contributed reporting from Boston.