Joe Biden (AP/Jose Luis Magana)

Labor unions are warning Vice President Joe Biden that his support for a major trade pact with Asia could make it difficult for them to support him if he runs for president.

A longtime favorite of unions, Biden has been skeptical of previous sweeping free trade deals, warning of risks to US jobs. Yet he's firmly supporting the Trans-Pacific Partnership, putting him at odds with most Democratic presidential candidates and jeopardizing his support from unions if he enters the race.

Biden planned to speak Wednesday to union officials and organizers, many of whom are mounting an aggressive campaign to block the pact, known as TPP. The White House workers summit will offer the first glimpse at how Biden will sell the deal to skeptics as Congress considers its approval.

"It's an unfortunate side-effect of this trade policy because Joe Biden is beloved in the labor community," said Chuck Rocha, a longtime labor organizer and Democratic consultant. "But they're really drawing a line in the sand on the issue '€” more than they ever have in the history of the movement."

The vice president "supports the TPP agreement and will help pass it on the Hill," his office said.

Biden, who is considering running for president, would be the only major Democratic candidate backing TPP. Hillary Rodham Clinton has said she's still studying the final deal, but said earlier this year she probably wouldn't have voted to grant President Barack Obama so-called "fast track" negotiating authority that makes it easier to gain congressional approval. Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley are staunchly opposed.

Biden has defended the deal publicly, and lobbied Democratic lawmakers last year to give Obama fast-track authority. He also worked to seal the deal abroad, calling for its completion during conversations with Japanese and Canadian leaders.

"This is a game changer," Biden said earlier this year in Mexico, describing TPP as a "comprehensive, high-standard trade agreement" that would raise the bar for 21st century trade.

Clinton and other Democrats are aggressively seeking union endorsements in the primary. Galen Munroe, a Teamsters spokesman, said opposition to TPP would be an important consideration. AFL-CIO labor federation strategic adviser Eric Hauser said defeating TPP is a key part of the higher-wages agenda his group will use to evaluate all candidates.

Designed to spur trade between Asia and the Western Hemisphere, TPP would alleviate more than 18,000 tariffs on US exports, Obama has said. After a review period, Congress will eventually hold an up-or-down vote on whether to ratify the deal.

As a senator, Biden voted for the North America Free Trade Agreement with Canada and Mexico, which labor groups detest, but later called for its renegotiation. During the George W. Bush administration, he opposed trade agreements with Central America, Singapore, Chile and Oman.

As vice president, Biden has won plaudits from unions for championing middle-class issues like higher wages. Union workers shouted "Run, Joe, run," as he marched in a Labor Day parade last month with AFL-CIO president Richard Trumka, who also met privately with Biden as he weighs a 2016 campaign.

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