WASHINGTON — An ambitious trade pact involving the United States and 11 other nations is a major victory for President Barack Obama but has the potential to create a rift among Democrats and cause friction with a key base of their political support, labor unions.

It will be weeks before the full scope of the agreement is known, but several labor groups are worried that it will result in American jobs being sent to counties with lower wages and less stringent labor and environmental standards. A congressional vote on the pact — the Trans-Pacific Partnership — is not expected to occur until well into next year, providing the unions the chance to maximize leverage with lawmakers coveting support.

The president has to wait 90 days before signing the pact, and only then will Congress begin the process of voting on it. Approval of the deal would give Obama a legacy-defining victory. He went to work immediately on Monday in selling the agreement and will visit the Agriculture Department today to tout its benefits for farmers.

“When more than 95 percent of our potential customers live outside our borders, we can’t let countries like China write the rules of the global economy,” Obama said in a statement released Monday.

To achieve a victory, Obama will need help from Republicans and will need to overcome doubts from a key Democratic constituency. In the hours after the trade deal was announced, some union leaders made clear that a candidate’s stance on the Trans-Pacific Partnership will serve as a critical test in determining whether he or she can expect support.

While unions have lost political clout as their numbers have declined, their political action committees donated more than $60 million to political campaigns during the 2012 elections, with about 90 percent of that money going toward Democratic candidates, according to the Center for Responsive Politics.

Chris Shelton, president of the Communications Workers of America, whose members include customer service reps and computer technicians, said the union will “hold accountable those members of Congress who support this giveaway to the 1 percent.”

Tom Buffenbarger, president of the International Association of Machinists and Aerospace Workers, said, “This is a very, very serious and dangerous proposal, and those who would see fit to simply fall for another trade deal, thinking it is good for America, they will incur our dedicated effort to unseat them.”