"We will build a great wall along the southern border, and Mexico will pay for the wall, 100 percent. They don't know it yet, but they're going to pay for it."

So said Republican presidential nominee Donald Trump in a combative speech on immigration policy last week, just hours after he met with Mexican President Enrique Pena Nieto.

Pena Nieto says he told Trump that Mexico would not pay for Trump's proposed wall along the southern border. The wall is the hallmark of a race-baiting campaign strategy that helped Trump win the GOP nomination. (Trump insists the issue of paying for the wall didn't come up during the closed-door confab with the Mexican president.)

The meeting backfired on Pena Nieto, who has faced withering criticism in its wake from the Mexican press. And now opposition politicians in the country's national legislature are proposing a plan to retaliate against the U.S. if Trump is elected in November and follows through on the wall and his threats to scrap the North American Free Trade Agreement.

The long-shot bill, from Sen. Armando Rios Piter, would require a hard-eyed review of Mexico's various treaties with the U.S. This includes the 1848 Treaty of Guadalupe Hidalgo, which transferred California and much of Arizona, New Mexico, Nevada and other Western states from Mexico to the United States after a war between the two countries.

The bill also includes a harsh tax on Americans transferring money from Mexico to the U.S., among other economic sanctions.

"In cases where the property/assets of (our) fellow citizens or companies are affected by a foreign government, as Donald Trump has threatened, the Mexican government should proportionally expropriate assets and properties of foreigners from that country on our territory," the bill states.

"This is the first step towards establishing a public policy about how Mexico should react in the face of a threat," Rios Piter said in The Hill newspaper.

Former Mexican legislator Augustin Barrios Gomez, who supports the proposed bill, says that Mexico must hit back hard if the U.S. tries to revamp NAFTA and pushes other policies that hurt Mexico's economy.

"The idea is that the relationship between the two countries is structured on friendship and trust. To break a treaty, originally proposed by the United States, now so important to Mexico, would be seen as an act in bad faith and the relationship would have to be rethought," Barrios Gomez said.

Trump has not yet responded to the bill, which, if Mexico passed it and then found a way to abrogate the Treaty of Guadalupe Hidalgo and, even more fantastically, somehow retook its lost territory, would force Trump to put his wall on Oregon's southern border.

The GOP nominee has focused his ire recently on Arizona Sen. Jeff Flake, a Republican who opposes his candidacy -- in large part because of Trump's immigration proposals. In a tweet over the weekend, Trump referred to Flake as "very weak and ineffective... Sad!"

-- Douglas Perry