WASHINGTON — Sen. Ted Cruz spent Monday in South Texas, where the idea of a massive border wall is deeply unpopular and widely viewed as unnecessary.

Like President Donald Trump, the Texas Republican argues that violence along the border, and especially on the Mexican side, justifies the construction project.

One claim that Cruz often repeats is that the city of Juárez, across the Rio Grande from El Paso, is so dangerous that it had more than 3,000 murders just last year, and that the wall separating those cities is the main reason for El Paso's low rates of violent crime.

In fact, last year, Juárez had fewer than 800 murders.

"That's flat wrong," said Jeremy Slack, a professor at the University of Texas at El Paso who studies migration, the drug trade and border violence. The per capita murder rate across the river in Juárez is similar to rates in Baltimore, Chicago or New Orleans, "which is not great, but is not near the height of the 2010 drug war."

That's when drug lord Joaquín "El Chapo" Guzmán and his Sinaloa cartel went to war with rival cartels. With more than 3,000 murders, the morgues overflowed and Juárez gained the distinction of being the most murderous city in the world.

No one would argue that 800 murders in a city of 1.4 million makes Juárez an island of serenity. But it's dramatically less than 3,000 — a claim Cruz has continued to make even after inquiries from The Dallas Morning News for data to back it up.

The campaign has not provided any documentation.

"It's certainly a politics of fear," Slack said. "Yes, these are very horrible things. Yes, they're scary and yes, they're dangerous. But they're not after people in the United States, because those are their clients. You don't want to cause problems for the people that are buying your merchandise. That's bad for business."

Here's how Cruz presented the argument Thursday during a rally in Arlington, painting his rival, Rep. Beto O'Rourke, as weak on border security:

"So where is Beto on immigration? Really bad. Not only does he oppose a wall but he says we have too many border walls and fences already, and he wants to tear down the border walls we already have," the senator said.

"El Paso has a relatively low crime rate. You know what El Paso also has? A wall. And you know what's on the other side of that wall? The city of Juárez. Last year in Juárez they had over 3,000 murders. So, 3,000 murders in Juárez, a wall protecting El Paso, and what does Beto want to do? Tear down that wall. As we say in Texas, that boy there don't think right," Cruz said.

1 / 8Mexican police carry a dead man from a garage in Ciudad Juarez where five corpses were found on March 30, 2010. (JESUS ALCAZAR / Getty Images) 2 / 8Mari Carresco visited the grave of her son, Javier Ramos Carresco, who was killed in a massacre at a nightclub in 2009, in Ciudad Juarez, Mexico, on Nov. 3, 2013. (2013 File Photo / The New York Times) 3 / 8 A police officer takes notes at an early morning murder on March 26, 2010 in Juarez, Mexico.(Spencer Platt / Getty Images) 4 / 8A helicopter of the Mexican police overflies Ciudad Juarez, on April 8, 2010. Northern Mexico was hard hit by drug-related violence that killed over 15,000 people during the prior three years despite a nationwide clampdown involving the deployment of over 50,000 troops.(JESUS ALCAZAR / Getty Images) 5 / 8Bullet holes on a window of funeral parlor, one of many businesses that were burned down for not paying a monthly tax charged by criminal gangs to thousands of local businesses in Ciudad Juarez, Mexico, on July 6, 2011.(JESUS ALCAZAR / Getty Images) 6 / 8Drug lord Joaquín "El Chapo" Guzmán was extradited to the United States on Jan. 19, 2017, and flown from a jail in Ciudad Juarez, Mexico, to New York to face a number of charges.(2017 File Photo / Attorney General of the Republic (PGR) of Mexico) 7 / 8Federal Police check a suspect of an attack against Federal Police in Ciudad Juarez, Mexico, on February 10, 2011. (JESUS ALCAZAR / Getty Images) 8 / 8Workers in El Paso replaced a section of the Mexico-U.S. border fence next to the international border bridge Sept. 26.(Herika Martinez / Agence France-Presse)

Cruz made the same case Saturday in Houston and at other stops across the state all last week. He stumps Monday with Utah Sen. Mike Lee in Harlingen, Mission and Laredo.

O'Rourke grew up in El Paso and represents the city in Congress.

Like most Democrats and even many Republican officeholders along the southwest border, he argues that a wall of the sort envisioned by Trump would waste tens of billions of dollars without adding meaningfully to the security of the United States.

He routinely makes the point that his hometown is the safest city in the United States, or close to it.

Precise statistics on murders in Mexican cities are hard to come by.

Attorneys general in Mexican states collect data on homicides that are investigated by authorities, but those statistics miss many killings that are reported in local news media, and by medical examiners.

México Evalúa, a public policy think tank that studies crime and corruption in Mexico, reported in early October that Juárez had 736 murders in the first nine months of 2018, well ahead of the 2017 pace, when the city had 642 murders for the entire year.

Molly Molloy, a librarian at New Mexico State University who tracks crime data for Mexico from a variety of sources, counted 772 murders in Juárez last year.

"Ted Cruz is wrong, completely. ... It's nowhere near as bad as 3,000," she said. "But he's right that it's a very violent place, and it's been getting worse since the turnaround in 2016."

By her estimates, the body count for Juárez in 2018 passed 1,000 in October, and is on pace to top 1,300 by year's end.

"It's still really bad," she said, "and it's still going up."

Molloy began tracking murders in Juárez during the hyperviolence of 2010. No one had ever seen such carnage. Federal and local authorities managed to tamp it down locally, though violence has spread nationwide and last year, Mexico had more murders — around 31,000 — than in 2010.

"The trend is not good," Molloy said.

Violent crime in the state of Chihuahua was 90 percent higher in August 2018 than in August 2017, and most of that was in Juárez.

Slack said the causes aren't entirely clear. It probably involves a power struggle within the Sinaloa cartel, and disputes between that cartel and others. The wall isn't what contains the killings to the Mexican side, though, in his view.

"If the wall were to come down tomorrow, I don't expect any major difference in the crime rate in El Paso," he said. "When people are fighting over who gets to sell the drugs, that's when there's a lot of violence. Nobody fighting over that in the United States ... but the wall doesn't really have anything to do with it. Most drugs come through the port of entry. The best way to get drugs across the border is mixed in with legal cargo."