As some 3,000 Hondurans made their way through Guatemala, attention turned to Mexico, after U.S. President Donald Trump threatened Thursday to close the U.S.-Mexico border if authorities there fail to stop them — a nearly unthinkable move that would disrupt hundreds of thousands of legal freight, vehicle and pedestrian crossings each day.

With less than three weeks before the Nov. 6 midterm elections, Trump seized on the migrant caravan to make border security a political issue and energize his Republican base.

"I must, in the strongest of terms, ask Mexico to stop this onslaught — and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!" Trump tweeted, adding that he blamed Democrats for what he called "weak laws!"

....In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!.. —@realDonaldTrump

The threat followed another one this week to cut off aid to Central American countries if the migrants weren't stopped. Trump made a similar vow over another large migrant caravan in April, but didn't follow through and it largely petered out in Mexico.

On Thursday, Mexico dispatched additional police to its southern border after the Casa del Migrante shelter on the Guatemalan side of the border reported that hundreds of Hondurans had arrived there.

Apparently pleased with that response, Trump retweeted a BuzzFeed journalist's tweet of a video clip showing the police deployment, adding his own comment: "Thank you Mexico, we look forward to working with you!"

The president also dispatched Secretary of State Mike Pompeo to meet with Mexican President Enrique Pena Nieto in Mexico City on Friday.

Trump has threatened to close the U.S.-Mexico border if a caravan of Honduran migrants is allowed into Mexico. (Edgard Garrido/Reuters)

Mexican officials said the Hondurans would not be allowed to enter as a group and would either have to show a passport and visa, which few have, or apply individually for refugee status, a process that can mean waiting for up to 90 days for approval. They also said migrants caught without papers would be deported.

Marcelo Ebrard, who is set to become foreign relations secretary when Mexican president-elect Andres Manuel Lopez Obrador takes office Dec. 1, said Trump's tweets need to be understood in the context of the upcoming U.S. midterm elections.

"The electoral process is very near, so he is making a political calculation," Ebrard said in an interview with Radio Centro.

Trump's stance, he said, was "what he has always presented," adding he saw "nothing surprising in it."

Only God on high can stop us. — Juan Escobar , Honduran migrant

Current Foreign Relations Secretary Luis Videgaray was also sanguine and viewed things through the lens of U.S. politics.

"Nobody likes [Trump's comments]. There's no reason to give them greater transcendence or importance," Videgaray said from the United Nations where he sought the world body's help processing asylum requests from the migrants. "What is important to us is the migrants, respect for human rights, their due protection, particularly the most vulnerable."

Mexican officials say the Hondurans will not be allowed to enter as a group, and will require passports or visas. (Edgard Garrido/Reuters)

Still, the idea that Mexico could close its porous southern border, or that the United States would choke off the lucrative trade and other traffic between the two nations, strained the imagination.

"There would be huge economic impacts for both the United States and Mexico ... but limited effect on illegal immigration," said Andrew Selee, president of the Migration Policy Institute.

"The president certainly can slow down crossing at legal border crossings where about a million people cross each day. That would really hurt legal transit between the two countries and manufacturing and trade, which would affect American workers," Selee said. "But it would have much less impact on illegal border crossings between ports of entry."

Juan Escobar, 24, said he had heard about Trump's comments but said they would not dissuade the migrants from continuing their journey.

"Only God on high can stop us," Escobar said.

Carlos Lopez, 27, said he was concerned by Trump's threats, but "you have to keep fighting."

Priority over new trade deal?

Trump also warned that he prioritizes border security over even the recently struck trade deal to replace NAFTA, the United States-Mexico-Canada Agreement, or USMCA.

"The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border," Trump tweeted.

Analysts didn't see the pact as being in imminent danger, though trade attorney Daniel Ujczo of Dickinson Wright PLLC said there is "a significant concern" Trump could hold the agreement hostage over future issues.

"Leaders around the world are skeptical that any deal with this U.S. administration is actually final," Ujczo said, "particularly one such as the USMCA where the ink has not been put to the signature line."

U.S.-bound migrant caravans have been going on for years, with travelling in numbers seen as offering protection from assaults, robberies, even shakedowns by police. They're also a cheaper alternative to the $7,000 to $10,000 US that smugglers charge for passage to the border.

Still, it wasn't until this year that the caravans received widespread attention.

There are 2,400 National Guard troops from four states deployed at various points along the U.S.-Mexico border. (John Gibbons/San Diego Union-Tribune via AP)

Vague about costs

Texas, New Mexico and Arizona sent hundreds of National Guard troops in the spring after Trump publicly mused about sending the military. California Gov. Jerry Brown, who has had a contentious relationship with the Trump administration, later agreed to send 400 of his state's National Guard troops. In September, he opted to extend their presence for six months.

The total number of National Guard troops from the four states is believed to be about 2,400. Defence Secretary James Mattis had authorized the use of up to 4,000 National Guard troops, extended now through September 2019, but with the provisions that they support U.S. Customs and Border Protection but not perform law enforcement duties or have contact with migrants at the border.

The administration has been vague about the costs associated with their deployment.