WASHINGTON (AP) — President Joe Biden hit the campaign trail to boost Democrats as crunch time ahead of the midterm elections kicked off, visiting the swing states of Wisconsin and Pennsylvania to mark Labor Day with trade unionists he hopes will turn out in force for his party in November.

“The middle class built America. Everybody knows that,” Biden told a workers’ gathering at park grounds in Milwaukee. “But unions built the middle class.”

Later Monday, he was flying to Pittsburgh — returning to Pennsylvania for the third time in less than a week and just two days after his predecessor, Donald Trump, staged his own rally in the state.

Trump spoke Saturday night in Wilkes-Barre, near Scranton, where Biden was born. The president made his own Wilkes-Barre trip last week to discuss increasing funding for police, decry GOP criticism of the FBI after the raid on Trump’s Florida estate and to argue that new, bipartisan gun safety measures can help reduce violent crime.

Two days after that, Biden went to Independence Hall in Philadelphia for a prime-time address denouncing the “extremism” of Trump’s fiercest supporters. On Monday, he’s attending Labor Day festivities in Milwaukee, in another key swing state, Wisconsin, before traveling to Pittsburgh for that city’s parade.

The unofficial start of fall, Labor Day also traditionally starts a political busy season where campaigns scramble to excite voters ahead of Election Day on Nov. 8. That’s when control of the House and Senate, as well some of the country’s top governorships, will be decided.

Trump has endorsed candidates in key races around the country and Biden is warning that some Republicans now believe so strongly in Trumpism that they are willing to undermine core American values to promote it. The president said Thursday that the midterms will be a battle “for the soul of the nation,” the same slogan he used to win the 2020 election, and that “blind loyalty to a single leader, and a willingness to engage in political violence, is fatal to democracy.”

Trump responded during his Saturday rally that Biden is “an enemy of the state.”

On Monday, Biden said “I’m not talking about all Republicans” but slammed “MAGA Republicans, the extreme right and Trumpies.” That referred to his Trump’s “Make America Great Again” campaign cry and Biden highlighted incidents like last year’s mob attack on the U.S. Capitol.

The crowd jeered as Biden chided Republican Sen. Ron Johnson of Wisconsin for voting against a Democratic-backed measure meant to lower prescription drug prices.

Biden told the Milwaukee rally that many in the GOP have “chosen to go backwards, full of anger, violence, hate, division.”

“But together we can, and we must, choose a different path forward,” Biden said. “A future of unity and hope. we’re going to choose to build a better America.”

The president also returned to another theme that was a centerpiece of his 2020 campaign, that labor unions were America’s economic engine.

“I come from the corporation state of the world,” Biden said, referring to his decades representing pro-financial business Delaware in the Senate. But he drew cheers declaring that he wasn’t anti-corporatation, but the country’s big businesses should “pay their fair share.”

Union endorsements helped Biden overcome disastrous early finishes in Iowa and New Hampshire to win the Democratic primary, and eventually the White House. He has since continued to praise labor unions — even though many voters without college degrees, many working class, remain among Trump’s strongest bloc of supporters.

Mary Kay Henry, president of the 2-million-member Service Employees International Union, called Biden championing unions heading into the midterms “critical” and said that the labor movement must “mobilize in battlegrounds across the country to ensure that working people turn out.”

“We’re really excited about the president speaking directly to workers about, if he had the opportunity, he’d join a union,” Henry said. She added: “This president has signaled which side he’s on. And he’s on the side of working people. And that matters hugely.”

Biden, meanwhile, has personal history with Pittsburgh’s Labor Day parade, which is among the nation’s largest. He attended the 2015 installment as vice president and returned in 2018. Both times, Biden, now 79, faced questions about whether he’d run for president in upcoming elections — which he opted against in 2016 before winning the White House in 2020.

This year, the oldest president in the nation’s history has faced speculation about if he’ll seek a second term in 2024 — though he’s insisted that’s his intention, and the pressure has dissipated some in recent weeks, amid a string of policy and political successes for Biden and his party.

Still, both perennial presidential battleground states Biden is visiting on Monday may provide key measures of Democrats’ strength before this November and 2024. With inflation still raging and the president’s approval ratings remaining low, how much Biden can help his party in top races remains to be seen.

That was on full display in Wisconsin, where Democratic Lt. Gov. Mandela Barnes is trying to unseat incumbent Republican Sen. Ron Johnson, but drew criticism from Johnson’s campaign for being noncommittal beforehand about appearing with Biden in Milwaukee. In the state’s other top race, Tim Michels, a construction executive endorsed by Trump, is attempting to deny Democratic Gov. Tony Evers a second term.

Evers was speaking at the labor event Biden addressed and also briefly greeted the president in a backstage photo line. Barnes did not and the president noted in his speech that the candidate “couldn’t be here today.”

“We have a president who understands the challenges facing working families,” Evers told the crowd. He said that Biden “hasn’t forgotten that working families matter, not just on Labor Day, but every single day of the year.”

Pennsylvania voters are choosing a new governor, with state Attorney General John Shapiro facing another Trump-endorsed Republican, Doug Mastriano, and a new senator. That race is between Democratic Lt. Gov. John Fetterman and Trump-backed celebrity heart physician Mehmet Oz. Shapiro and Fetterman both planned to attend Monday’s Pittsburgh parade.

The Pennsylvania and Wisconsin races could decide which party controls the Senate next year, while the winner of each governorship may influence results in 2024’s presidential election. The stakes are particularly high given that some Trump-aligned candidates have spread lies about widespread fraud that did not occur during the 2020 election — raising questions about what might happen if a candidate they don’t support wins the next presidential contest.

Vice President Kamala Harris marked Labor Day in Massachusetts, where at a breakfast meeting with the Greater Boston Labor Council, she said, “When union wages go up, everybody’s wages go up.”

“When union workplaces are safer everyone is safer,” Harris said. “When unions are strong, America is strong.”