Campaigning alongside his one-time running mate Kamala Harris in his birth state of Pennsylvania, President Joe Biden called his vice president the only "rational" choice in November's election against Republican Donald Trump.

Biden was met with chants of “Thank You, Joe” from hundreds of union members as he and Harris came onstage at an International Brotherhood of Electrical Workers hall in Pittsburgh.

He said choosing her to be vice president was the “single best” decision of his presidency and told the union members that electing her will be “the best decision you will ever make.”

Harris used the joint campaign appearance with Biden in the critical swing state on Monday to say that U.S. Steel should remain domestically owned — concurring with the White House's monthslong opposition to the company's planned sale to Japan's Nippon Steel.

That echoes Biden, who repeated Monday what he's said since March — that he opposes U.S. Steel's would-be sale to Nippon, believing it would hurt the country's steelworkers. It also overlaps with Republican former President Donald Trump. It's little surprise that Harris would agree with Biden on the issue, but it nonetheless constitutes a major policy position for the vice president, who has offered relatively few of them since Biden abandoned his reelection bid and endorsed his vice president in July.

"We know this is going to be a tight race till the very end," Harris told the Pittsburgh crowd.

The joint rally with Biden was Harris' second of the day and followed Pittsburgh's Labor Day parade, one of the country's largest. It was their first joint appearance at a campaign event since the election shakeup six weeks ago.

Harris opened her Labor Day campaigning solo with an event in Detroit, where hundreds of audience members wore bright yellow union shirts and hoisted "Union strong" signs. The vice president said "every person in our nation has benefited" from unions' work.

"Everywhere I go, I tell people, 'Look, you may not be a union member, you'd better thank a union member," Harris said, noting that collective bargaining by organized labor helped secure the five-day work week, sick pay and other key benefits and solidify safer working conditions.

"When unions are strong, America is strong," she said.

The 81-year-old Biden has spent most of his lengthy political career forging close ties with organized labor. The White House said he asked to introduce Harris in Pittsburgh — instead of the usual other way around — because he wanted to highlight her record of supporting union workers.

In addition to opposing the Nippon Steel sale, Biden has endorsed expanding tariffs on imported Chinese steel — another area of policy agreement with Trump, who has cheered steeper foreign tariffs on many imports. Still, in a statement Monday, U.S. Steel said it remains "committed to the transaction with Nippon Steel, which is the best deal for our employees, shareholders, communities, and customers."

"The partnership with Nippon Steel, a long-standing investor in the United States from our close ally Japan, will strengthen the American steel industry, American jobs, and American supply chains, and enhance the U.S. steel industry's competitiveness and resilience against China," the company said, noting that it employs nearly 4,000 people in Pennsylvania alone.

Nippon Steel reacted to Harris' comments by saying it was confident that its "acquisition of U. S. Steel will revitalize the American steel rust belt, benefit American workers, local communities, and national security in a way no other alternative can." The Harris campaign released a statement countering that sentiment from David McCall, president of the United Steelworkers union, who said Harris' opposition to the sale "once again made it clear that she will always stand up for steelworkers."

The 59-year-old Harris has sought to appeal to voters by positioning herself as a break from former president Trump's acerbic rhetoric while also looking to move beyond the Biden era. Harris events feel very different from Biden's, which usually featured small crowds. But the vice president's agenda includes the same issues he's championed: capping the cost of prescription drugs, defending the Affordable Care Act, growing the economy, helping families afford child care — and now her position on the sale of U.S. Steel.

The vice president has promised to work to lower grocery store costs to help fight inflation. She's moved faster than Biden in some cases, calling for using tax cuts and incentives to encourage home ownership and ending federal taxes on tips for service industry employees. But she's also offered relatively few specifics on major policies, instead continuing to side with Biden on top issues.

Harris appeared onstage with Biden after the president addressed the opening night of last month's Democratic National Convention, but they had not shared a microphone at a political event since Biden himself was running against Trump. At that time, the campaign was using Harris mostly as its chief spokeswoman for abortion rights, an issue they believe can help them win in November as restrictions grow and health care worsens for women following the fall of Roe v. Wade.

For more than 3 1/2 years, Harris has been one of Biden's chief validators. Now the tables are turned, as Harris looks to lean on Biden — a native of Scranton, Pennsylvania — to help win the potentially decisive state.

Although the vice president has appeared more forceful in speaking about the plight of civilians in Gaza, as Israel's war against Hamas there nears the 11-month mark, she also has endorsed Biden's efforts to arm Israel and bring about a hostage deal and ceasefire. Before she left Washington for Detroit, Biden and Harris met in the White House Situation Room earlier Monday with the U.S. hostage deal negotiating team.

"History will show what we here know: Joe Biden has been one of the most transformative presidents," Harris said in Pittsburgh. "And as we know Joe still has a lot of work to do."

When that event was over, Biden and Harris rode back to the airport together in the presidential limo. Air Force One and Air Force Two subsequently took off within moments of each other to return to suburban Washington — though the president and vice president never travel on the same plane for continuity of government reasons, just in case of an air emergency.

Before their Pittsburgh event, Harris joined Biden on Monday in the Situation Room to meet with the U.S. hostage deal negotiating team to discuss their continuing efforts on a deal that would secure the release of the remaining hostages.

The two leaders "received an update from the U.S. negotiation team on the status of the bridging proposal outlined by the United States, Qatar, and Egypt" and "discussed next steps in the ongoing effort to secure the release of hostages," per the White House.