Pittsburgh, Pennsylvania - President Joe Biden and Vice President Kamala Harris campaigned together for the first time Monday, in a public display of teamwork after she replaced him as candidate and revived Democratic election hopes.

Seeking to attract voters in the key battleground state of Pennsylvania, the pair met union members in Pittsburgh before standing side-by-side to speak to about 600 supporters.

Biden repeatedly name-checked Vice President Harris, who is in a close race to beat Donald Trump to the White House on November 5.

"We have made a lot of progress, and Kamala and I are going to build on that progress, and she's going to build on it," Biden said.

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"I'll be on the sidelines, but I'll do everything I can to help."

The Labor Day event at the International Brotherhood of Electrical Workers union underlined how Biden will play a role in the campaign in the final months before the vote.

Biden (81) initially resisted bowing out of the presidential race after his disastrous debate performance against Trump, but he has since swung behind Harris (59).

"There's only one person you have a rational choice with this time – that's Kamala Harris," he said to cheers.

"I know her. I trust her," he added. "She has a backbone like a ramrod, and the compass of a saint."

"This woman knows what she's doing. I promise you, if you elect Kamala Harris as president, it will be the best decision you will have ever made."

Riding a wave of enthusiasm from supporters, Harris has held packed rallies across the country and raked in cash donations over recent weeks.

Polls show her entry improving the Democrats' chances at defeating Republican Trump, but with the outcome highly uncertain.

In Pittsburgh, Biden and Harris embraced and held hands before she paid him fulsome praise.

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"Joe Biden has been one of the most transformative presidents in the United States that we have ever witnessed, and it comes from his heart," she said.

"Joe and I talk a lot about the fact that we are so proud to be the most pro-union administration in America's history."

"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 claims about his 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 has yet to fully define her platform, the vice president has sought to promise change – but has faced scrutiny for pivoting from progressive positions she ran on when competing against Biden for the Democratic nomination in 2020, including Medicare For All.

Harris had also previously pledged to ban fracking – a key issue in Pennsylvania – but now has ruled that out.

The vice president has also faced strong public outrage over the administration's continued military and diplomatic support of Israel, which has killed more than 40,000 Palestinians since October. Polling data indicate that a failure to change course may cost Harris at the ballot box.

Pennsylvania is one of a handful of states that will decide the election, and is possibly the one on which the whole result will hinge.

Monday's joint appearance with Biden also launched the two-month sprint to the vote, with Labor Day marking the traditional end of the US summer.

"Friends, 64 days until the most important election of our lives, and probably one of the most important in the life of our nation," Harris said, describing herself as the underdog in the race.

"We have some hard work then ahead of us. But here's the beauty of us in this room – we like our work. Hard work is good work."

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