Joe Biden and Kamala Harris on Monday made their first post-convention joint appearance on the presidential campaign trail, celebrating Labor Day with a tribute to union workers in Pittsburgh.

“We are so proud to be the most pro-union administration in American history,” Harris said. “I love Labor Day. I love celebrating Labor Day, and Pittsburgh is the cradle of the American labor movement.”

Between comments about the administration’s support for organized labor and Donald Trump’s attacks on labor organizing, Harris, the vice-president, spoke against the pending purchase of US Steel by Nippon Steel, arguing that the iconic Pennsylvania steel company should remain in the hands of American owners.

“US Steel is an historic American company and it is vital for our nation to maintain strong American steel companies. And I couldn’t agree more with President Biden: US Steel should remain American-owned and American-operated.”

The United Steelworkers union, representing about 10,000 US Steel employees, opposes the $14.9bn deal, taking issue with Nippon Steel’s alleged violations of the union’s rights concerning change of control under their four-year basic labor agreement signed in 2022. The union and the companies are in arbitration talks.

Harris again voiced support for the Protecting the Right to Organize (PRO) act, a broad basket of labor reforms that would spur union organizing.

Kenny Cooper, president of the IBEW union, introduced Biden and Harris, noting that the passage of the Butch Lewis Act by Harris’s tie-breaking vote saved the benefits of two million union members. “They were only tied up for one reason,” he said. “We couldn’t find a Republican senator.”

Harris also cast the tie-breaking vote for the Inflation Reduction Act, which the USW International president, David McCall, said in comments had been “revolutionizing the cement, chemical, glass and steel sectors along with other traditional core industries”.

Donald Trump, the Republican presidential nominee, also opposes the Nippon Steel deal and has said he will block it if president. Biden announced his opposition to the Nippon Steel deal in March.

Less crisply perhaps than Harris, Biden described the accomplishments of his administration in Pennsylvania, from investments in clean energy to infrastructure money. He noted that his administration required project labor agreements that respected labor rights and required American products, while reminding listeners that Donald Trump appointed union busting officials to the National Labor Relations Board.

“Wall Street didn’t build America,” Biden said. “The middle class built America and unions built the middle class.”

The appearance of Biden and Harris together provides an image of how the two may campaign in the waning days of the election. Biden described Harris as having “the backbone of a ramrod and the moral compass of a saint”.

Harris spent the morning in Detroit, hailing the virtues of union organizing – the five-day work week, sick leave, vacation time and other benefits – with labor leaders at Northwestern High School.

“We celebrate unions because unions helped build America, and unions helped build America’s middle class,” she said. “When union wages go up, everybody’s wages go up.”

Biden is the first sitting president to walk a union picket line, supporting the United Auto Workers in their dispute with major car manufacturers in September 2023. “You guys – the UAW – you saved the automobile industry back in 2008 and before,” Biden shouted through a bullhorn on the picket line in Michigan. “You made a lot of sacrifices, gave up a lot. The companies were in trouble. Now they are doing incredibly well and guess what? You should be doing incredibly well too.”

Shawn Fain, president of the UAW, has been both a strident voice reinvigorating the American labor movement and a strident opponent of Trump. “Donald Trump is all talk, and Kamala Harris walks the walk,” Fain said at the Democratic national convention in August, while wearing a shirt that said “Trump is a scab”. Harris supporters chanted that phrase in Detroit this morning.

Though Trump called in his acceptance speech before the Republican national convention for Fain to be “fired immediately”, the Republican nominee has made overtures to labor voters during his run to return to office. The renegotiation of the North American Free Trade Agreement and his proposed tariffs of 10-20% on foreign trade have been central to his outreach, arguing that this will bring manufacturing back from offshore plants.

But Project 2025 – a conservative playbook for a second Trump administration penned by the Heritage Foundation – aims to end merit-based employment for thousands of unionized federal workers; calls for changes to “protected concerted activity” which would allow employers to retaliate more easily against union organizing; and eliminate the “persuader rule” requiring company disclosures when hiring union-busting consultants.

Trump has also flip-flopped in public comments about the electric car industry, initially calling for an end to electric car mandates but recently walking that rhetoric back after the Tesla CEO, Elon Musk, endorsed his candidacy. During an interview on Musk’s X/Twitter social media space, Trump gushed at Musk’s approach to labor relations.

“They go on strike,” said Trump. “I won’t mention the name of the company, but they go on strike and you say, ‘That’s OK. You’re all gone. You’re all gone. So every one of you is gone,’ and you are the greatest.”

That prompted the Teamsters president, Sean O’Brien – who spoke at the RNC convention to the surprise of many labor leaders – to walk his own overtures towards Trump back. “Firing workers for organizing, striking and exercising their rights as Americans is economic terrorism,” O’Brien said.