Harris eyes endorsement in high-stakes meeting with only major union that hasn't endorsed her campaign

With over 1.3 million members, the International Brotherhood of Teamsters is one of the country's 10 largest unions.

And the Teamsters are the only union among the nation's top 10 that hasn't endorsed Vice President Kamala Harris in the 2024 presidential election.

On Monday, Harris makes her pitch to the Teamsters, as the Democrats' presidential nominee works to maintain her party's traditional high level of support from organized labor while she faces off with former President Trump.

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Vice President Kamala Harris speaks during a campaign rally at Bojangles Coliseum, in Charlotte, North Carolina, Thursday, Sept. 12, 2024. (AP Photo/Jacquelyn Martin)

Teamsters General President Sean O’Brien late last year announced the union’s first-ever interview process for their 2024 presidential endorsement and invited the major party candidates to make their cases.

The Teamsters met earlier this year with Trump and separately with President Biden, whom Harris succeeded atop the Democrats' national ticket two months ago.

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"We look forward to having a conversation on the direction of the country and the issues that matter to working people," O'Brien said earlier this month, as he pointed toward the meeting with Harris.

O'Brien made history in July as he became the first Teamsters president to address a Republican National Convention.

General President of the International Brotherhood of Teamsters Sean O'Brien speaks during the Republican National Convention in Milwaukee, Wisconsin, July 15, 2024. (Andrew Caballero-Reynolds/AFP via Getty Images)

But his speech also sparked controversy, as he drew the ire of some other top Teamsters leaders and some of the rank-and-file membership.

Democrats didn't invite O'Brien to address their convention last month in Chicago.

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A week before the Democrats' convention, O'Brien argued that Trump was guilty of "economic terrorism" after the former president – in an interview with billionaire business mogul Elon Musk – praised the tech CEO for firing organizing workers.

"Firing workers for organizing, striking, and exercising their rights as Americans is economic terrorism," O'Brien emphasized in a statement.

Former President Trump departs a campaign event at the Central Wisconsin Airport on Sept. 7, 2024, in Mosinee, Wisconsin. A recent poll has Trump trailing Democratic nominee Vice President Kamala Harris in the battleground state. (Scott Olson/Getty Images)

Union workers were once reliable voters for Democratic candidates. 

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But thanks to his populist and nationalist messaging during his three White House runs, Trump has made major inroads with working-class voters.

Democrats continue to secure endorsements from organized labor leaders. Most other major unions other than the Teamsters backed Biden earlier this election cycle, and transferred their support to Harris after the president ended his 2024 campaign.

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.