“It’s much harder to connect actual provisions of the U.S.M.C.A. to cash for the wall, since they don’t put money in the coffers of the Treasury,” said Gary Hufbauer, a senior fellow at the Peterson Institute for International Economics. “Any connection between labor, auto rule of origin and other chapters to the wall is pretty remote.”

Scott Lincicome, a trade expert at the libertarian Cato Institute, said he viewed Mr. Trump’s claim with “immense skepticism,” given that the new agreement merely updates Nafta and that none of the changes will be a major revenue driver. He also noted that “the biggest amount of liberalization is on the Canadian side” — not Mexico’s — through gradually opening Canadian markets to American dairy imports.

Mr. Trump may be suggesting that the new agreement will increase economic activity in the United States, therefore generating enough tax revenue to offset the cost of his wall. The new trade deal is intended to make it less lucrative for American manufacturers to locate production in Mexico by requiring higher wages for autoworkers — a provision that the Trump administration says will shift more car production back to the United States.

But even if that happens, higher economic output in the United States “is not coming at the expense of Mexico. It’s a trade growth effect,” said Mr. Lincicome.

It also remains to be seen just how much the U.S.M.C.A. would contribute to economic growth, and how much revenue it would generate. Most trade agreements actually lead to lower federal revenue from tariffs, according to the Congressional Budget Office, but it’s unclear whether their net effect on federal budgets is positive or negative. Still, any potential increase in federal revenue would come from American taxpayers, not Mexico.