GRAND RAPIDS, Mich. (AP) -- Vice President Kamala Harris rallies in Michigan's union halls, standing alongside the state's most powerful labor leader, while former President Donald Trump fires back from rural steel factories, urging middle-class workers to trust him as the true champion of their interests.

As they compete for blue wall states with deep union roots, the presidential candidates are making their case to workers in starkly different terms. And nowhere is that contrast more significant than in Michigan, where both candidates are vying for workers' support in a race that could mark a pivotal moment for organized labor.

"The American dream was really born here in Michigan," United Auto Workers President Shawn Fain told a crowd of several hundred while campaigning for Harris in Grand Rapids. Fain, who described Michigan as "sacred ground" for his union at the early October rally, warned that the dream was on "life support" and that unions like his were key to protecting it for American workers.

Harris, who is planning to meet with union workers again in Michigan on Friday, hopes her message -- amplified by supporters such as Fain -- will resonate beyond the union families that once formed a rock-solid base for the Democratic Party. Her campaign has grown increasingly concerned about her standing with men in the blue wall states of Michigan, Wisconsin and Pennsylvania, where they are looking to union leaders to help mobilize voters in a political landscape that has shifted in the winds of a rapidly changing economy.

These concerns intensified recently when Harris failed to secure two key union endorsements that in 2020 went to President Joe Biden, who has touted himself as the most labor-friendly president in U.S. history. The International Association of Firefighters and the International Brotherhood of Teamsters both declined to endorse anyone, with the Teamsters citing a lack of majority support for Harris among their million-plus members.

The Teamsters have traditionally been less reliably Democratic than other unions, having endorsed Republican Presidents Richard Nixon and Ronald Reagan in the past. Some state-level unions have also diverged from their national leadership, with Michigan's Teamsters and California's main firefighters' union backing Harris.

Still, any break in unity within the labor movement could strike a blow against a party that has worked hard to restore unions as a central source of its power at the ballot box.

"When you talk about unions, you're addressing more than just unionized workers. Most people in states like Michigan have a family member or close friend in a union," said Adrian Hemond, a longtime political strategist in Michigan. "Unions are just a vessel to get that messaging out to workers."

Trump has seized on the union non-endorsements, claiming they prove rank-and-file workers support his vision for the country.

Many Midwestern communities once core to the labor movement have shifted to the right in recent decades, often in response to economic concerns such as deindustrialization and the removal of trade barriers. In that same span, non-college-educated white voters across the country began voting more conservatively for a number of reasons, including concern about cultural issues involving race and gender.

In Michigan, home to the Big Three automakers and the largest concentration of UAW workers, Trump seeks to capture an even larger share of these votes by framing Harris as a supporter of electric vehicle mandates and trade policies that he says send jobs overseas.

Attempting to separate union workers from their leaders, he labeled Fain a "stupid idiot" and praised Tesla CEO Elon Musk for firing workers who went on strike. The UAW says that could intimidate people who work for the Trump campaign or at Tesla who might want to join a union.

In 2020, Biden narrowly carried the blue wall states that had broken with Democrats in 2016 for the first time in decades on his way to winning the White House. That election win was built on a foundation of strong support from unionized voters, who have traditionally formed a turnout machine for Democrats in the Midwest. But it stood apart from past Democratic victories in a number of significant ways.

While Trump narrowly won white voters in Michigan in 2020, the former president's vote margin was highly polarized along educational, professional and income lines; Trump won nearly two-thirds of non-college-educated white voters in the state, while Biden and Trump were drawn to a near tie among college-educated white voters, according to AP VoteCast, a comprehensive survey of the electorate.

Among Michigan's nonwhite voters, who make up 16% of the state's electorate, Biden won a resounding 80% of the vote. But signs of that coalition fracturing have emerged more recently, particularly among Arab Americans in metro Detroit, many of whom are expected to turn away from Democrats due to the Biden administration's handling of the Israel-Hamas conflict.

As Trump again seeks the presidency, his campaign hopes to boost GOP support among the state's non-college-educated white and nonwhite workers to unprecedented levels, partly to offset expected losses Trump will face with white college-educated voters, where he has hemorrhaged support since his 2020 loss and subsequent efforts to overturn the results in Michigan, Pennsylvania and other swing states.

"I think that part of the problem that Democrats are having with some of the white male, blue collar voters is not within the union itself," said Brian Rothenberg, a former UAW spokesman. "It's those folks that are children or relatives of union members that just aren't doing as well."

Harris has aimed to win over these voters by emphasizing how unions benefit all workers. At a Labor Day rally in Detroit, she said "you better thank a union member" for the five-day work week, for sick and paid leave and for vacation time.

"When union wages go up, everybody's wages go up," said Harris.

Just over a year after securing new contracts for UAW workers at Ford, General Motors and Stellantis, Fain has staked much of his political capital -- and potentially his future -- on supporting Harris. He argues that UAW backing for Democrats has remained steady over recent elections, with approximately 60% of members voting for the Democratic presidential nominee.

Biden became the first president to walk the picket line when he visited Michigan in late 2023 amidst the autoworker strike. A day later, Trump traveled to Michigan and appeared at a non-union plant, where he railed against Biden's electric vehicle push and told workers to "get your union leaders to endorse me, and I'll take care of the rest."

Union leaders have said his first term was far from worker-friendly, citing unfavorable rulings from the nation's top labor board and the U.S. Supreme Court, as well as unfulfilled promises of automotive jobs. They emphasize Democratic achievements in states like Michigan, including the recent repeal of a union-restricting right-to-work law enacted over a decade ago by a Republican-controlled legislature.

With membership dwindling in states like Michigan, Fain will need to attract more than just union workers to secure a victory for Harris, who has campaigned in the state alongside him. If the union president cannot deliver Michigan after all these efforts, it could raise questions about his union's political influence in future elections.

"This is a generation-defining moment, where we are right now," Fain told Michigan voters. "This election is going to determine where we go."

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Cappelletti reported from Lansing, Michigan. Associated Press writer Tom Krisher in Detroit contributed to this report.