PITTSBURGH (Reuters) -Democratic presidential candidate Kamala Harris said on Monday that U.S. Steel should remain in domestic hands, making a pitch alongside President Joe Biden to working-class voters in Pennsylvania who are also being courted by her rival.

The event on Labor Day, a U.S. holiday that signifies the start of the post-summer sprint to the Nov. 5 election, marked Harris and Biden's first appearance together at a campaign rally since she officially became the Democratic nominee.

Harris used campaign events on Monday in Michigan and Pennsylvania, two battleground states, to court the crucial labor vote. At a packed union hall in Pittsburgh, she echoed Biden's concern over U.S. Steel Corp being acquired by Japan's Nippon Steel.

"U.S. Steel is a historic American company, and it is vital for our nation to maintain strong American steel companies," Harris said at the rally. "U.S. Steel should remain American-owned and American-operated."

Biden said Harris would build on the progress they had made during his administration to improve the lives of union workers and that he would do everything he could to help.

"I'll be on the sidelines," he said.

The Harris campaign has sought to appeal to union workers in much the same way as Biden has throughout his presidency. The Democrats aim to keep union workers from being tempted to vote for former Republican President Donald Trump, who also has blue-collar appeal.

At the rally in Pittsburgh, Todd Hamer, 48, said he thought Biden's support for labor would help Harris.

"She's new," Hamer said. "We're all still learning (about her), but she does have a deep history and I think she's going to continue to stand by labor leaders and unions to help support unions and their cause."

Trump, who is locked in a tight race with Harris, stayed off the campaign trail on Monday. His campaign has announced several events for later in the week, including in North Carolina and Wisconsin.

The Harris campaign mocked Trump's lack of public appearances on Monday. "Trump ... golfing?" the Harris campaign said on X.

A Trump campaign aide said Harris' decision to rally voters with Biden gave the Trump team another opportunity to tie the Democrats together politically.

Trump has said he would move to block the U.S. Steel deal, a potential merger that has stirred anxiety among some unionized workers, a key voting bloc in Pennsylvania and the other "Rust Belt" swing states likely to determine the results of the election.

Responding to Harris' criticism, U.S. Steel said it was committed to the deal with Nippon Steel. Both companies aim to close the deal by the end of the year depending on regulatory approvals.

In emailed comments, Nippon Steel said that acquisition of U.S. Steel will revitalize the American steel rust belt, benefit local workers and communities along with national security "in a way no other alternative can."

"We believe that a fair and objective regulatory review process will support this outcome, and we look forward to closing the transaction as soon as possible," it said.

GAZA WAR CASTS SHADOW

Both Harris and Trump are expected to ramp up outreach to voters in the coming weeks, especially in swing states that could prove decisive in the election.

Harris is hoping to keep up the enthusiasm her entry into the race has sparked among Democrats, who are donating record amounts of money and volunteering by the tens of thousands.

A recent Reuters/Ipsos poll found Harris was leading Trump nationally 45% to 41%.

On Monday, developments in the Middle East over talks on a deal to reach a Gaza ceasefire and release hostages cast a shadow on campaigning.

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 Gaza ceasefire strategy.

Biden told reporters earlier on Monday he did not think Israeli Prime Minister Benjamin Netanyahu was doing enough to secure a hostage deal.

Late on Monday, Biden said he would "eventually" talk to Netanyahu but declined to specify when.

Some 1,200 Israelis were killed in Hamas' surprise attack on Oct. 7 and around 250 were taken hostage, according to Israeli tallies.

Health authorities in Gaza say more than 40,000 Palestinians have been killed in the Israeli assault on the enclave since that has also displaced nearly its entire 2.3 million population, caused a hunger crisis and led to genocide allegations at the World Court that Israel denies.

Biden and Harris also met with the U.S. hostage negotiation team in Washington on Monday and discussed the next steps in efforts to free the remaining captives.

The war is weighing on the U.S. election, with pro-Palestinian activists threatening to ramp up protests against Harris on the campaign trail. Thousands of pro-Palestinian activists opposing U.S. support for Israel's war in Gaza held a protest in New York City on Monday.