United States President Joe Biden has appeared at a campaign event with Vice President and Democratic nominee Kamala Harris for the first time since dropping out of the presidential race.

The pair spoke at a rally in Pittsburgh, Pennsylvania, on Monday to mark the Labor Day holiday. They hope to shore up support from unions and blue-collar workers.

Pennsylvania is considered a key battleground in the November 5 election, in which Harris will face off with former President and Republican candidate Donald Trump. The 81-year-old Biden was the presidential candidate until July when he abruptly dropped out of the race following a debate performance that raised concerns over his age.

Speaking on Monday, Biden promised he would “be on the sidelines” and “do everything I can to help” in the final stretch of the race. He condemned Trump as anti-union, saying, “He’d rather cross [a picket line] than walk one.”

“I have no problem walking the picket line,” said Biden, who became the first US president in history last year to join striking workers on a picket line. “Neither does Kamala.”

“I know her. I trust. The first decision I made as nominee in 2020 was selecting her as my vice president,” he said. “It was the single best decision I made as president of the United States of America”.

Harris, meanwhile, led a chant of, “Thank you Joe” before pledging to continue Biden’s legacy as the most “pro-union administration in US history”.

Echoing Biden, she also said she would work to keep the country’s steel production under US control.

In March, Biden said industrial giant US Steel Corp, which has agreed to a takeover by Japan’s Nippon Steel for $14.9bn, must remain a domestically-owned US firm.

“We will continue to strengthen America’s manufacturing sector,” Harris said.

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

Harris and Biden appeared together at the Democratic National Convention and at a White House event on Medicare drug price cuts last month. However, the event in Pittsburgh was their first joint appearance at a campaign rally since Harris officially became the nominee.

Earlier in the day, the vice president held an event in Detroit, Michigan – another key battleground state – where she again faced protesters calling for Washington to immediately shift its material and political support for Israel amid its ongoing war on Gaza.

‘Thread this very fine needle’

Reporting from Pittsburgh, Al Jazeera’s Phil Lavelle said that while Harris wanted to benefit from her association with Biden, she also wanted people to know she was different from him.

“She has to thread this very fine needle between being seen to be associated with Joe Biden, being linked to his perceived achievement… because Joe Biden, remember, was the first president to walk a picket line. He’s seen as very pro-union,” he said.

“At the same time, she has to carve out her own direction and be seen as a change candidate, which is what she’s trying to project herself as, a candidate for change,” Lavelle said.

“Now this is something that, of course, the other side is saying is not true. They’re saying that she is part of the current administration. How can she be a changed candidate?”

Jeremy Zogby, an independent pollster, said it will be particularly important for Harris to draw a distinction on the economy, which has been central to Republican attacks on the Biden administration.

Zogby added that while some polls suggest Harris is leading Trump nationally and in key battleground states, much could change before election day. Harris and Trump will face off in their first debate on September 10.

“Battleground states are called battleground states because the needle is always moving,” he told Al Jazeera.

“Right now, we’re kind of leaving this wave of the Kamala honeymoon, where there’s been a focus on personality.

“I sense that that’s coming to an end and the issues are going to come back into the forefront. Between now and election day, you are going to have a lot of things happen.”