The Teamsters, one of the nation's largest labor unions, voted unanimously on Tuesday to withhold its endorsement of Democratic presidential front-runner Hillary Clinton.

In a closed-door meeting on Tuesday in Charleston, S.C., the group's board voted 26-0 to hold off on endorsing Clinton, according to Fox News' James Rosen.

The vote was reportedly "an intentional snub" of Clinton in response to her voicing opposition to the construction of the Keystone XL pipeline, which she previously supported, notes Breitbart.

A second reason the union is holding back is because it wants to first sit down with Republican candidates, "most notably, front-runner Donald Trump, who has collaborated with unionized work forces across his real estate career," Rosen said. Trump's recent tax reform proposal could be appealing to some Teamsters as it drastically lowers corporate and individual tax rates for the middle class in an attempt to encourage job creation, savings and investment.

Rosen added that the union is also waiting to see if Vice President Joe Biden announces a bid.

"It's a very significant development, not least because the Teamsters is such a big, powerful and wealthy union. You're talking about well over a million unionized workers, a lot of them in key states," Larry Sabato, a University of Virginia political scientist, told Fox News. "It's one more piece of evidence that Hillary Clinton's efforts to lock down the nomination have utterly failed."

As for the largest labor union in the country - the National Education Association - rumors are circulating that they may soon hand out an early endorsement for Clinton, although the move has many state leaders and members planning to protest the endorsement, according to Politico.

Clinton has scored three other major union endorsements: the International Union of Bricklayers and Allied Craftworkers, the American Federation of Teachers and the International Association of Machinists and Aerospace Workers, reports The Hill.

Last month, the National Nurses Union chose to endorse Sen. Bernie Sanders, I-Vt., over Clinton.

And if Clinton doesn't withdraw her support of President Obama's Trans-Pacific Partnership trade deal, the president of The American Federation of Labor and Congress of Industrial Organizations recently hinted that she may lose their support too.