Former Rep. Beto O’Rourke, a prospective Democratic presidential contender, made waves with a video ad attacking President Donald Trump’s proposed U.S.-Mexico border wall. His claims in the social media ad, seen more than 5 million times since it came out a week ago, are mostly accurate.

A look at some of the Texan’s statements in the ad:

O’ROURKE AD: “Since 2007, the undocumented immigrant population has grown more through visa overstays than unauthorized border crossings.”

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THE FACTS: This claim is correct. It does not, however, address the argument of the wall’s proponents that the wall would be effective in slowing the illegal entry of hundreds of thousands of people from the south.

While the U.S. is struggling to deal with an uptick of families and children seeking asylum from Central America, the number of border crossings has actually declined in recent years. Meanwhile, the number of foreigners who did not leave the country when they were supposed to — commonly called visa overstays — has stayed consistent. Temporary visas are issued to foreigners who want to work, study, travel or seek medical treatment in the country.

There was a high of 1.6 million border apprehensions in 2000, and that dropped to about 310,000 in the 2017 budget year . A 2017 Homeland Security report found that the number of “known got aways”— an estimate Border Patrol agents developed — fell from 600,000 in 2006 to roughly 106,000 in 2016.

In contrast, Homeland Security found that 700,000 foreigners who came by plane or ship overstayed their visa from October 2016 to September 2017. The department has not consistently tracked how many foreigners overstayed their visas in recent years.

Visa overstays are making up a larger share of immigrants coming to the U.S. illegally every year, according to the Center for Migration Studies, a New York-based think tank . Overstays accounted for only 34 percent of illegal entries into the U.S. in 2004 but by 2014 they made up 66 percent of new entries. The study estimates 42 percent of the 11 million immigrants believed to be living in the U.S. illegally as of 2014 had overstayed their visa.

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O’ROURKE AD: “President Trump’s proposed border wall would ... exile hundreds of thousands of acres in the U.S. to a no man’s land between the river and the wall.”

THE FACTS: It’s true that some land would be rendered useless and inaccessible. It’s unclear how much territory would be involved and O’Rourke did not substantiate his claim that it would be hundreds of thousands of acres.

Existing portions of fencing along the U.S.-Mexico border already have relegated some property, in some cases a road or a property owner’s yard, to “no man’s land” space between U.S. barriers and Mexico.

South Texas is especially vulnerable to this because of the curve of the Rio Grande River — the fifth longest river in North America, not the fourth longest, as O’Rourke’s ad asserts.

Josiah Heyman, director of the Center for Inter-American and Border Studies at the University of Texas at El Paso, estimates that at least several thousand acres of U.S. lands would be cut off if the wall is built.

“There are already many examples of land being cut off,” Heyman said. “We don’t how much more this would apply to.”

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O’ROURKE AD: “President Trump’s proposed border wall would ... seal off critical wildlife corridors.”

THE FACTS: Scientists are indeed concerned that new walls or fences could hurt wildlife populations along the border.

The border has a number of diverse wildlife populations, among them jaguars and exotic birds, according to Tim Keitt a biology professor at the University of Texas. Construction of a wall could restrict mobility for some wildlife, cut other populations by half and destroy habitats, he said.

“There are species that are not very common and they happen to live right in the pathway where people want to erect barriers,” Keitt said.

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Associated Press writers Nomaan Merchant in Houston and Colleen Long in Washington contributed to this report.

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