President Donald Trump is demanding that Democrats agree to include $5 billion in a spending bill to fund construction of a wall along the US-Mexico border.

Otherwise, he said, he would be "proud" to shut down the government.

At the same time, Trump continues to insist that Mexico will pay for the border wall.

Trump says money coming in from a new trade agreement means Mexico will effectively be paying for the wall.

Senate Minority Leader Chuck Schumer pointed out the logical flaws in this argument during a speech.

President Donald Trump's newest argument about who exactly is paying for the wall along the US-Mexico border doesn't make a lot of sense.

The latest iteration of Trump's argument is that money coming in from the new USMCA trade deal — the revised North American Free Trade Agreement — will fund the wall.

"Mexico is paying (indirectly) for the Wall through the new USMCA, the replacement for NAFTA!" Trump tweeted Wednesday. "Far more money coming to the US."

Trump first made the claims during a meeting with Senate Minority Leader Chuck Schumer and House Speaker-Designate Nancy Pelosi on December 12 and reiterated it in a tweet last week.

"I often stated, 'One way or the other, Mexico is going to pay for the Wall,'" Trump tweeted. "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!"

But this argument is in direct conflict with Trump's demands in the latest government-shutdown fight, according to Senate Minority Leader Chuck Schumer. Schumer lambasted Trump's arguments, pointing out that they don't logically make sense.

Trump has said the US must allocate billions of dollars for a wall along the US-Mexico border. But as Schumer pointed out, that comes into direct conflict with the president's insistence that Mexico will pay for the wall.

But Schumer asked a simple question during a floor speech last week: If Mexico, as Trump insists, is paying for the wall, why does the US have to spend $5 billion on it in a new bill?

"Well, Mr. President, if you say Mexico is going to pay for the wall through NAFTA, which it certainly won't, I guess we don't have to," he said. "Let's fund the government."

Trump seems to be arguing that the USMCA will boost US gross domestic product, thus increasing the amount American businesses and families earn. That increased income for Americans would then boost tax revenue, since higher incomes means more money for taxes.

But even in that scenario, the funds are still coming from tax revenue — which are paid by Americans, not Mexico.

"If the president really believes what he tweeted this morning that his new NAFTA would pay for the wall, he wouldn't be threatening to shut down the government unless American taxpayers pay for the wall," Schumer said. "He can't have it both ways."

The only way to have Mexico directly pay for the wall through the USMCA, without taking money collected from American taxpayers, would be to include a clause in the trade deal. No such clause exists.

The USMCA is identical to NAFTA in many major ways, and economists don't expect a large boost to US GDP from the deal.