Democratic presidential candidate Kamala Harris says US Steel should remain in domestic hands, while making her pitch to working-class voters in Pennsylvania during a rally with President Joe Biden.
The Labor Day rally marked their first appearance together on the campaign trail since Harris replaced Biden at the top of the Democratic ticket in July.
The president introduced the vice president to a crowd of more than 600 people packed into a union hall.
"US Steel is a historic American company, and it is vital for our nation to maintain strong American steel companies," Harris said in her remarks at the rally.
"US Steel should remain American-owned and American-operated," she added.
The Labor Day holiday marks the start of the vital post-summer sprint to the November 5 election.
Both Harris and her Republican rival, former president Donald Trump, are expected to ramp up outreach to voters, especially in battleground states which could prove decisive such as Pennsylvania, Georgia, Michigan and Nevada.
Trump stayed off the campaign trail on Monday, while Harris held events in Detroit and Pittsburgh, where she used her remarks to underscore support for steel workers.
Harris' position on US Steel Corp mirrors that of Biden, who said in March that the company, which has agreed to be bought by Japan's Nippon Steel for $US14.9 billion ($A22 billion), must remain a domestically owned American firm.
Trump in February had signalled he would move to block the deal, a potential merger that 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. Last month, Trump said he would halt the deal.
US Steel said on Sunday it is committed to the deal with Nippon Steel, which it calls the "best deal for them (employees) and the communities where they live in".
Developments in the Middle East over talks on a deal to reach a Gaza ceasefire and release hostages cast a shadow on the event on Monday.
Over 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, sparking sharp criticism of the Biden administration's ceasefire strategy and new pressure on Israeli Prime Minister Benjamin Netanyahu to bring the remaining hostages home.
Biden told reporters 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.
Biden and Harris met with the US hostage negotiation team on Monday and discussed the next steps in efforts to free the remaining captives, the White House said.
Meanwhile, Trump will participate in a FOX town hall on Wednesday hosted by Sean Hannity and later this week will address the Fraternal Order of Police at their fall meeting in Charlotte, North Carolina, and hold a rally in Wisconsin.
A recent Reuters/Ipsos poll found that Harris was leading in the race against
