WASHINGTON—President Trump has asserted that Mexico will pay for a wall along the southern U.S. border, through increased trade even if its leaders have refused to do so directly. Trade experts are trying to figure out how that would work.

The president’s request to Congress for $5 billion to build a portion of the wall has triggered a spending battle that has partially shut down the government. But Mr. Trump, who campaigned in 2016 on a promise that Mexico would pay for the wall, is pushing back on concerns that U.S. taxpayers ultimately will bear that burden.

With Mexico’s new president, Andrés Manuel López Obrador, echoing his predecessor’s refusal to fork over a penny for the wall, the White House is asserting that the costs to U.S. taxpayers will be compensated by savings and revenue generated from the newly signed U.S.-Mexico-Canada trade agreement, known as USMCA.

“Mexico is paying (indirectly) for the Wall through the new USMCA, the replacement for NAFTA!” Mr. Trump wrote on Twitter earlier this month. “Far more money coming to the U.S. Because of the tremendous dangers at the Border, including large scale criminal and drug inflow, the United States Military will build the Wall!”

It is a complicated pitch to make to voters, and one that faces hurdles to carry out—including persuading the new Democratic House majority in Congress to approve the revised trade agreement.