US President Joe Biden and Democratic candidate Kamala Harris have campaigned together for the first time, pitching to working-class voters at a rally in Pennsylvania.

The Labor Day rally was the pair’s first appearance together on the campaign trail since Harris replaced Biden at the top of the Democratic presidential election ticket in July.

Biden introduced Harris to a crowd of more than 600 people packed into a union hall.

“I’ll be on the sidelines, but I’ll do everything I can to help,” he said. “Are you ready to make Donald Trump a loser again?”

Harris used the event to speak out against US Steel’s proposed purchase by Japan’s Nippon Steel.

“US Steel is a historic American company, and it is vital for our nation to maintain strong American steel companies,” Harris told them.

“US Steel should remain American-owned and American-operated.”

Harris’ position on US Steel Corp mirrors that of Biden, who said in March that the company, which has agreed to be bought by Nippon for $US14.9 billion ($A22 billion), must remain a domestically owned American business.

The potential merger has stirred anxiety among some unionised workers, a key voting bloc in Pennsylvania and the other “rust belt” swing states likely to determine the results of the election.

In February, Trump had signalled he would move to block the deal while last month he said he would halt it.

On Sunday, US Steel said it was committed to the deal with Nippon Steel, which it calls the “best deal for them (employees) and the communities where they live”.

Monday’s Labor Day holiday marks the start of the vital post-summer sprint to the November 5 presidential election.

Both Harris and her Republican rival, former president Donald Trump, are expected to ramp up outreach to voters, especially in battleground states that could prove decisive. They include Pennsylvania, Georgia, Michigan and Nevada.

Trump stayed off the campaign trail on Monday, while Harris held other events in Detroit and Pittsburgh, where she also spoke about her support for steel workers.

However, developments in the Middle East over talks on a deal to reach a Gaza ceasefire and release hostages cast a shadow over Monday’s events. 

At the weekend, Israel recovered the bodies of six hostages from a tunnel in Gaza where it said they were recently killed by Palestinian Islamist group Hamas. The developmet has sparked sharp criticism of the Biden administration’s ceasefire strategy and piled more pressure on Israeli Prime Minister Benjamin Netanyahu to bring the remaining hostages home.

Biden said on Monday he did not think Netanyahu was doing enough to secure a hostage deal.

The war is weighing on the US election, with pro-Palestinian activists threatening to ramp up protests against Harris on the campaign trail.

Thousands of pro-Palestinian activists opposing US support for Israel’s war in Gaza held a protest in New York City on Monday.

The White House said Biden and Harris met the US hostage negotiation team on Monday, to discuss the next steps in efforts to free the remaining captives.

Meanwhile, Trump will participate in a FOX town hall on Wednesday hosted by Sean Hannity. Later this week, he will address the Fraternal Order of Police at their autumn meeting in Charlotte, North Carolina, and hold a rally in Wisconsin.

-with AAP