President Donald Trump on Monday characterized a growing migrant caravan as a “national emergency,” even as Catholic leaders in Mexico urged their country to help the Central Americans reach the United States.

“Sadly, it looks like Mexico’s police and military are unable to stop the Caravan heading to the Southern Border of the United States,” Trump tweeted ahead of a Make America Great Again (MAGA) campaign rally scheduled for Monday evening in Houston.

The president wrote that criminals and “unknown Middle Easterners” are among the throng, and he added that he has alerted the U.S. military and Border Patrol “that this is a National Emergy [sic].”

Trump also lamented that he has been unsuccessful in persuading Congress to close loopholes that make it easier for illegal immigrants to stay in the country while pursuing meritless asylum claims or awaiting deportation court dates.

“Every time you see a caravan, or people illegally coming, or attempting to come, into our country illegally, think of and blame the Democrats for not giving us the votes to change our pathetic immigration laws! Remember the midterms! So unfair to those who come in legally,” the president wrote.

Trump also renewed his previous threats to take punitive action against Guatemala, Honduras and El Salvador over their failures to stop the latest caravan.

“We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them,” he wrote.

The caravan launched from Honduras a little more than a week ago with an estimated 1,300 people. Although estimates now vary, it seems clear the numbers have grown substantially.

The Washington Post on Sunday quoted one of the group’s organizers estimating that the total numbers of 5,600. The Los Angeles Times, citing Gerardo Hernandez — head of the local government’s emergency services in Mexico — reported that 7,233 people on Sunday had registered at a shelter in Ciudad Hidalgo.

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The caravan may be as large as 10,000 men, women and children, Fox News reported.

“You can’t even walk, there’s just so many people,” Hernandez told Reuters. “So far, they’re all peaceful, thank God.”

Guatemalan President Jimmy Morales claimed that 2,000 people in the caravan have been returned to their home countries.

The caravan is certain to be a prime topic for Trump as he stumps Monday night for Sen. Ted Cruz (R-Texas), who is locked in a surprisingly competitive election fight with Democrat Beto O’Rourke.

More than 100,000 people asked for tickets to the event, prompting the Trump campaign to announced a “Big Texas Tailgater” outside the Toyota Center in Houston for those unable to fill one of the arena’s 19,000 seats inside.

The Mexican government has tried to process Central Americans in an orderly fashion, registering asylum claims and issuing 45-day visitor visas to allow some of the migrants to remain lawfully in Mexico.

But several news organizations reported that many of the migrants grew impatient with that process and crossed over other parts of the porous border between Mexico and Guatemala.

Cuarto Poder, a local newspaper in southern Mexico, reported that Catholic leaders on Sunday demanded that the Mexican government help the migrants. Jaime Calderón Calderón, the bishop of Tapachula — where many of the Central Americans slept on the streets — called on federal, state and municipal authorities in Mexico not to block the men, women and children determined to reach America.

“The caravan of Honduran migrants claims humanity,” Calderon said, according to the newspaper.

Cuarto Poder also reported that Archbishop Fabio Martínez Castilla of Tuxtla Gutiérrez warned Mexican officials not to adopt the stance of the United States. Tuxtla Gutierrez is the capital of the Mexican state of Chiapas.

He said the Hondurans are “fleeing unsustainable living conditions” and deserve assistance.

“Nobody leaves their home, their land and their heritage for pleasure, who does it with the hope of finding new and better opportunities for life,” he said.

The archbishop added, “We beg the Mexican authorities, whose functions are still valid, to review migratory policies with a human heart, as well as not forgetting to meet the needs of Chiapans themselves, among them the most vulnerable.”

Related: Migrant Caravans Are a ‘Disgrace to the Democrat Party,’ Says Trump

Mexican police could do little to stop travelers from coming across the border, the Associated Press reported on Sunday. Some were openly defiant and — confirming Trump’s warning about criminals — acknowledged that they had committed crimes.

“We’re warriors. We got to get to the place we got to get to,” said Luis Puerto, a 39-year-old Honduran. “We’re gonna keep on going and we’re not gonna stop.”

“We are going to get to the border of the U.S.,” he said. “I am not going to stop. I don’t care if I die.”

Puerto told AP he was determined to make it to North Carolina, where his wife and two daughters live. He said he had been deported from the United States after trouble with the law that he did not describe in detail.

“We are going to get to the border of the U.S.,” he said. “I am not going to stop. I don’t care if I die.”

Under those circumstances, Puerto is barred from re-entering the United States. He almost certainly could not qualify for asylum.