President Donald Trump speaks following a ceremony signing the America's Water Infrastructure Act of 2018 into law in the Oval Office at the White House in Washington, Oct. 23, 2018. Credit: Manuel Balce Ceneta | AP

WASHINGTON — President Donald Trump claimed Thursday that “money we save” from a new trade deal with Mexico and Canada would make good on his long-standing promise to have Mexico pay for a new southern border wall.

Trump’s questionable assertion, in a morning tweet, comes as he is lobbying Congress for $5 billion to help fund construction of the wall and threatening a partial government shutdown if he does not get his way.

In recent days, as the debate over the wall has come to the fore, Trump has faced renewed criticism for appearing to have abandoned his campaign pledge to make Mexico pay for it.

“I often stated, ‘One way or the other, Mexico is going to pay for the Wall,’” Trump wrote on Twitter. “This has never changed. Our new deal with Mexico (and Canada), the USMCA, is so much better than the old, very costly & anti-USA NAFTA deal, that just by the money we save, MEXICO IS PAYING FOR THE WALL!”

Mexican officials have said there was no discussion in the trade-deal negotiations of mechanisms under which Mexico would pay for the wall.

Trump spent much of the 2016 campaign promising American voters that Mexico would somehow pay for the construction of a wall along the U.S. border, a demand that angered Mexican officials but enthralled his supporters. Since becoming president, though, Trump has sought U.S. taxpayer money to fund the wall’s construction and threatened to shutdown parts of the U.S. government if lawmakers don’t acquiesce.

Last month, Trump and the leaders of Canada and Mexico signed documents that would rework the 1994 North American Free Trade Agreement. The new deal, which Trump has coined the USMCA, still must be approved by Congress before it takes effect, and some Democrats have signaled they will demand changes.

But Trump is claiming that this new trade deal, even though the outcome is uncertain, would somehow pay for the wall, an assertion that has stumped many budget experts.

“Boy, this is a stretch,” said William Hoagland, the former Republican staff director of the Senate Budget Committee.

Hoagland said the only reasoning he could contemplate to back up Trump’s Twitter post Thursday is that if the U.S. economy grows because of the new trade deal, Trump could claim the new tax revenue is a bonus and therefore is somehow related to Mexico.

But he said the same reasoning could be made to say Canada is paying for the wall’s construction. And Hoagland also said that none of this new money would actually come from Canada or Mexico. Also, Hoagland could not see a scenario that would have the USMCA “save” taxpayer money, like Trump asserted in his Twitter post.

“At the end of the day, the American taxpayer is still paying for it,” he said. “Because where are the revenues coming from? They are not coming from Mexican taxpayers.”

Trump made the same argument on Tuesday during a contentious Oval Office meeting, during which House Minority Leader Nancy Pelosi, D-California, urged the president not to pursue a partial government shutdown. Pelosi instead urged Trump to take a deal that would provide $1.3 billion for border fencing by extending current levels of funding.

Trump raised the idea that some money for the wall could come from the newly renegotiated North American trade agreement, an idea that Pelosi dismissed.

Washington Post writer David J. Lynch contributed to this report.