In his State of the Union address in February, President Trump said the North American trade agreement his administration had negotiated would reverse the flow of auto manufacturing jobs by ensuring “that more cars are proudly stamped with our four beautiful words: Made in the U.S.A.”

But with that deal, the United States-Mexico-Canada Agreement, now almost certain to become law, some analysts and unions are voicing skepticism that Mr. Trump will be able to make good on that grand promise.

“It’s not at all clear that there is going to be a positive effect on jobs in the auto industry,” said Mary Lovely, a senior fellow at the Peterson Institute for International Economics and a professor of economics at Syracuse University. “This is the hard lesson of economics, which is basically there’s a lot of factors here.”

The trouble, she and other experts said, is that the very provisions in Mr. Trump’s deal aimed at lifting employment could drive up the cost of making cars. That, in turn, could reduce demand and jobs. As a result, the pact’s net result on jobs could be a wash.