Minnesota Gov. Tim Walz addressed a firefighters union in Boston on Wednesday as part of an effort by the Democratic ticket to bolster support from organized labor as former President Donald Trump also tries to attract working-class voters.

Mr. Walz, who was picked last month as Vice President Kamala Harris’ running mate, spoke to the International Association of Firefighters. It’s one of the nation’s largest labor unions, with 350,000 fire, emergency, medical and rescue workers across the U.S. and Canada.

A former teachers union member, Mr. Walz told the crowd that he and Ms. Harris “will have your back” if they win the election in November.

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“When we’re in office we will make sure you have all the resources and protections you need to do your job and your service is respected and you come home safe every night,” Mr. Walz said.

He pointed to several efforts by the Biden administration to bolster firefighters’ pay and make their jobs safer. Mr. Walz said Ms. Harris worked closely with President Biden to get those bills over the finish line while overseeing the most pro-union administration in history.

During his remarks, Mr. Walz blasted Mr. Trump and his running mate, Sen. J.D. Vance of Ohio, accusing them of ignoring the working class and lining their own pockets off their hard work. 

“The only thing these guys know about working people is how to take advantage of them, how to not pay them,” he said.

Ms. Harris and Mr. Walz are highlighting their records supporting organized labor during campaign stops following the Democratic National Convention. Labor unions are a critical Democratic voting bloc, and Mr. Trump made inroads with rank-in-file members while winning the presidency in 2016.

Despite endorsements from labor leaders in Pennsylvania, Wisconsin and Michigan, Hillary Clinton lost those swing states that year because union members voted for Mr. Trump.

Mr. Biden won back some of those voters in 2020, helping him claim those three states. Ms. Harris is seeking to capitalize on that effort.

She plans to spend Labor Day campaigning before union groups in critical Rust Belt states. She will begin her day in Detroit, then join Mr. Biden in Pittsburgh.