Washington DC - Kamala Harris and President Joe Biden hit the campaign trail together for the first time Monday, in a public display of unity after she replaced him as candidate and revived Democratic election hopes.

To mark the Labor Day holiday, the pair were set to attend an event with union members in the key battleground state of Pennsylvania.

Biden (81) bowed out of the presidential race in late July under mounting pressure after his disastrous debate performance against Donald Trump.

His rapid endorsement of Vice President Harris (59) saw her shore up party support and she became the formal Democratic nominee just weeks later.

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Riding a wave of fresh enthusiasm, she has held packed rallies in key swing states across the country and raked in cash donations for the final two-month stretch of the campaign.

Polls show her entry improving the party's chances at defeating Republican Trump – but with the race still neck and neck.

Before her event with Biden in Pittsburgh, Pennsylvania, Harris spoke to another union crowd in Detroit, Michigan, seeking to boost support from working-class voters that Trump claims as his base.

Flanked by leaders of the national teachers and autoworkers unions, Harris told the Detroit crowd that Trump "intends to pull us back, including back to a time before workers had the freedom to organize."

"We're not going back!" the crowd chanted, using one of her campaign slogans.

Trump took the weekend off from campaigning and did not have public events scheduled Monday.

"For a candidate that claims to be rallying the support of workers, why is Donald Trump M.I.A. on Labor Day?" the Harris campaign needled in a statement.

The Republican, in a post on his Truth Social platform, touted his first-term economic record and accused Harris and Biden of having "undone all of that."

Harris last appeared with Biden after his speech two weeks ago at the Democratic National Convention in Chicago.

As she works to define her platform, the vice president has sought to promise change, while avoiding criticism of Biden's tenure.

On the issue of organized labor, she has tied herself closely to Biden.

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"The most pro-labor administration in history, under President Biden and Vice President Harris' leadership, support for union membership has grown to its highest level in half a century," her campaign said in a statement.

Harris has faced scrutiny for pivoting from the liberal positions she ran on when competing against Biden for the Democratic nomination in 2020.

Harris had previously pledged to ban fracking – a major source of income in Pennsylvania – but now has ruled that out.

Pennsylvania is one of a handful of states that will decide the election and is viewed as possibly the one on which the outcome will hinge.

In Pittsburgh on Monday, Harris and Biden will speak at a union hall where they will meet local members.

Harris is expected to say that Pittsburgh-headquartered US Steel – which Japan's Nippon Steel is seeking to buy – should remain domestically owned, a campaign official said.

Monday's joint appearance with Biden is also seen as launching the two-month sprint to the November vote, with Labor Day marking the traditional end to the US summer.

Earlier Monday, Harris joined Biden at the White House for a briefing on hostages and ceasefire negotiations in the Israel-Hamas war.

Six hostages were killed in Gaza over the weekend, including a US citizen.

Harris's running mate, Minnesota Governor Tim Walz, separately campaigned in Milwaukee, Wisconsin on Monday.

Walz posted to X on Monday wishing Americans a happy Labor Day

"As two middle-class kids, [Kamala Harris and I] know firsthand that the middle class built this country," he said. "And unions built the middle class. Happy Labor Day, America."