Cochise County, Arizona — Sen. JD Vance, former President Donald Trump's vice presidential running mate, toured the U.S.-Mexico border in Arizona on Thursday, where he criticized the Biden administration's immigration policies and targeted Vice President Kamala Harris over her record on the issue.

"It's hard to believe, until you see it with your own eyes, just how bad the policies of the Kamala Harris administration have been when it comes to the southern border," Vance said, not mentioning President Biden and focusing his criticism on the new presumptive Democratic nominee. The Ohio Republican received a tour of the border wall by law enforcement officials in Cochise County in Arizona, which has become a hotspot for border crossings.

Vance promised that a Trump-Vance White House would reimplement Trump's "Remain in Mexico" policy, stop so-called "catch and release" policies and finish the border wall. He also vowed to carry out mass deportations of illegal migrants, a promise that Trump has made since he launched his 2024 campaign.

The visit comes as the number of unlawful migrant crossings along the U.S. southern border has continued to fall, dropping for the fifth consecutive month in July and reaching the lowest level since the fall of 2020. But Vance told CBS News in an interview after his border tour that he did not think the Biden-Harris administration was doing anything right on immigration.

"No, I don't think they are. And when the Harris administration says the border crossings have dropped, she's not counting a lot of the people" who are arriving through legal ports of entry, Vance said. "And so actually, if you combine everything together, the number of people who are coming across our border illegally is still really at all-time highs."

Republicans often accuse the administration of playing a "shell game" by admitting migrants at ports of entry, diverting them from crossing illegally. But entering at a port of entry is not illegal, contrary to what Vance suggested, and the number of crossings has still decreased significantly when illegal crossings and admissions at ports of entry are grouped together.

When pressed what specifically he would do to curb the root causes of immigration from South American countries, something that Harris was tasked to do early on in the Biden administration, Vance falsely claimed that she was the "border czar," a misrepresentation of her role in addressing migration chiefly through diplomacy and private sector investment.

"Well, she was tasked as border czar with addressing the root causes of immigration," Vance said. "I think the root causes of illegal immigration are that Kamala Harris refuses to do her job." 

"This is not rocket science," Vance added, emphasizing he'd work to "build up the economies of Central America and South America to make migration less common so that people can build a life in their home countries."

In the first four months of 2024, Border Patrol recorded more than 250,000 migrant apprehensions in the Tucson sector, which includes Cochise County. That was the most of any region patrolled by the agency, according to government statistics. 

Vance also referenced his own family history with addiction as he blamed the Biden-Harris administration for the flow of fentanyl into the U.S.

"I have been a little kid waiting at the bedside of his mother, angry that his mom took something that she shouldn't have taken, but praying to God, please, Jesus let her wake up. And the unfortunate truth is because of the poison that Kamala Harris has let come into this country," Vance said. 

The Ohio senator also defended his push to finish the border wall from critics who say the barriers are disruptive and impractical.

"I'm sure that not everything's perfect, but law enforcement sometimes does cause disruptions, [and] it's the sort of thing that's totally necessary in order for us to have a secure border," Vance told CBS News. "I think the disruptions of having a flood of people come in is much worse than the disruption of having a wall. And so it's one of those things where the cost-benefit analysis, I think, definitely counsels in favor of us finishing this border wall."

Camilo Montoya-Galvez, Ed O'Keefe, Anne Bryson and Grace Kazarian contributed to this report.