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Vice President Kamala Harris’ campaign held a series of Labor Day-themed events in Michigan, Wisconsin and Pennsylvania on Monday, marking the unofficial start of fall campaigning in states that will be crucial to the November election.

The events come as Harris seeks to build her momentum over the next two months. While the Democratic presidential nominee has sought to expand the map of states in which she is competitive, the Labor Day stops highlight the importance of winning the three “blue wall” states that helped President Joe Biden win in 2020.

Biden joined Harris at a rally in Pittsburgh on Monday night, the president’s first joint campaign appearance with the vice president since he abandoned his re-election effort and endorsed Harris.

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

Harris used the event in Pennsylvania to declare that US Steel should remain domestically owned and that she was against the purchase of the company by the Japanese corporation Nippon Steel.

“The president has said it: U.S. Steel is a long-standing American company, and it is vital to our country to maintain strong American steel companies. And I completely agree with President Biden: U.S. Steel should remain American owned and operated,” Harris told a crowd of union members.

In a statement, a Nippon Steel spokesman defended the deal, saying that “US Steel and the entire American steel industry will be on a much stronger footing as a result of the acquisition.”

Harris’ remarks in Pennsylvania were largely similar to an earlier event in Detroit, where she spoke to a crowd of union members and leaders about the “dignity of work” and promised to strengthen collective bargaining protections, drawing sharp contrasts with her opponent, Donald Trump.

“We celebrate unions because unions helped build America and unions helped build the American middle class,” she said in a packed high school gymnasium here in Michigan.

Harris was joined on stage by national union leaders, including United Auto Workers President Shawn Fain, American Federation of Teachers President Randi Weingarten, and National Education Association President Becky Pringle.

The vice president reiterated her support for the PRO Act, legislation that would guarantee workers the right to organize and collectively negotiate workplace changes, and vowed to “end union busting once and for all.”

Monday’s events come 64 days before Election Day and, as Harris noted in her remarks, 24 days before mail-in voting begins in Michigan and 14 days before it begins in Pennsylvania. The vice president reiterated previous warnings that the campaign is nearing its end.

“I’m telling you, we know how they play, we know what they do,” she said. “So we’re not paying too much attention to the polls. We should know that, as Labor always does, we’re running here as if we’re the underdogs in this race because we know what we’re fighting for.”

Meanwhile, her running mate, Minnesota Governor Tim Walz, proudly boasted about his union ties at a union rally in Milwaukee. He celebrated Labor Day by declaring he was “in the pocket” of unions and urging Republicans to “take the damn risk” if they wanted to criticize his support of unions.

Walz discussed his union membership as a former public school teacher, his support of unions in Congress and as governor of Minnesota, and highlighted Harris’ support of unions.

Trump and his running mate, Ohio Senator JD Vance, had no appointments on Monday. Trump used his social media page to criticize Harris over high gas and food prices and touted his work in renegotiating America’s trade agreement with Mexico and Canada.

“We cannot continue to live under this weak and failed ‘leadership,'” Trump said on Truth Social.

The Harris campaign criticized the former president for not holding any events on the holiday.

“Donald Trump is failing workers on Labor Day because he is an anti-worker, anti-union extremist who, if he comes to power, will betray working families for his billionaire donors,” campaign spokesman Joseph Costello said in a statement.

Ahead of Harris’ Labor Day speech in Detroit, Michigan Governor Gretchen Whitmer praised the vice president’s achievements and accused Republicans of being out of touch with the common people.

“If your most famous phrase is ‘You’re fired,’ then you certainly don’t understand workers,” Whitmer said later, referring to Trump’s catchphrase from his former reality show “The Apprentice.” “I want our next president to tell workers, whoever she may be, I’m behind them.”

Whitmer said Harris was part of the “most pro-union administration in U.S. history,” praising Harris’ work as senator and attorney general, standing with striking workers and standing up to big banks and pharmaceutical companies, as well as Walz’s commitment to infrastructure investments in her state and raising the minimum wage for delivery drivers.

Trump criticized union leaders, including the United Auto Workers’ Fain union, but sought to rally support from workers in the same Rust Belt states where Harris and Walz campaigned on Monday. The former president turned his attention to the Biden administration’s efforts to boost production and purchases of electric vehicles, which Trump said would come at the expense of autoworkers.

Trump’s allies in Michigan railed against Harris on the issue of auto production ahead of the Democratic presidential candidate’s Labor Day visit to Detroit.

“Part of what makes America great is our economy, and the Harris administration is targeting autoworkers’ jobs,” Republican Rep. Lisa McClain of Michigan said Monday morning in a conference call with reporters. “There will continue to be layoffs.”

McClain, who represents the northern part of the Detroit metropolitan area, and Republican Rep. John James, who represents a neighboring district, both argued that the Biden-Harris administration’s policies to increase electric vehicle production in the U.S. are unrealistic.

The Harris team has refuted previous claims by Trump and Vance, including that the vice president supports a “mandate” for all electric vehicles. The administration’s goal is for electric vehicles to make up half of all new car sales by 2030.

This story has been updated with additional reporting.

CNN’s Samantha Waldenberg and Aaron Pellish contributed to this article.