Who are the Teamsters, and how could they influence the US election?

The United States' second largest labor union, the Teamsters, has chosen not to endorse either Kamala Harris or Donald Trump for the White House in November's election.

Could the decision by the powerful body impact the vote? And can the fact that several local branches in key swing states have defied that consensus to throw their support behind Harris make any difference?

- Who are the Teamsters? -

Officially known as the International Brotherhood of Teamsters, the union -- whose members number some 1.3 million -- was formed in 1903.

Since then, it has expanded from representing truck drivers to -- according to its own figures -- speak for some 10 percent of American union members, in sectors including agriculture, law enforcement, healthcare and aviation, negotiating for them and striking when necessary.

According to its website, a unionized private sector employee earns on average 38 percent more than a non-unionized one, and receives 54 percent more benefits.

- What do they think of the candidates? -

The leadership said Wednesday that it would not endorse a candidate as there was no majority among its membership in favor of Democrat Harris or Republican Trump.

Neither candidate has made serious commitments about their support for labor, the organization's president, Sean O'Brien, argued in a statement.

But in the wake of the announcement, many local branches rallied behind Harris -- notably, ones in swing states such as Nevada, Wisconsin, Michigan and Pennsylvania, which observers say will be key to victory under the US electoral college system.

- Who have they supported previously? -

The organization has supported every Democratic candidate for the White House since Al Gore in 2000.

Before that, it supported Republicans Ronald Reagan in 1984 and George H.W. Bush in 1988, before endorsing Democrat Bill Clinton in 1992.

In 1996, when Clinton faced off against Republican Bob Dole, it abstained.

- What does this mean for Harris and Trump? -

"It's undoubtedly a setback (for Harris) because it will allow Trump to claim working class, if not union, support," says Nelson Lichtenstein, a specialist in the history of labor relations at the University of California.

For Paul Clark, a Pennsylvania State University professor specializing in corporate labor relations, the abstention comes as a "disappointment" to Harris -- but "it's not all that important."

Support at the national level could be invaluable to the candidates because it activates the grassroots, he explained -- and in such a tight election, that could make the difference.

And yet, Harris has the backing of other major unions which, in addition to the support from local Teamsters branches, could soften the blow.

"In the long run, the endorsement of the local unions is going to be more valuable and have more of an impact on the outcome of the election," he said.

"It might pull Harris over the finish line."

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