WASHINGTON (AP) — Vice President Kamala Harris will visit the U.S.-Mexico border in Arizona on Friday as her campaign tries to turn the larger issue of immigration from a liability into a strength and hopes to counter a line of frequent, searing political attacks from former President Donald Trump.

Two people familiar with the matter confirmed the trip but insisted on anonymity on Wednesday to confirm details that had not been announced publicly.

Trump has built his campaign partly around calling for cracking down on immigration and the southern border, even endorsing using police and the military to carry out mass deportations should he be elected in November. Harris has increasingly tried to seize on the issue and turn it back against her opponent, though polls show voters continue to trust Trump more on it.

Just how important immigration and the border are ahead of Election Day was evidenced by Trump wasting little time reacting to word of Harris’ trip. He told a rally crowd in Mint Hill, North Carolina, that Harris was going to the border “for political reasons” and because “their polls are tanking.”

“When Kamala speaks about the border, her credibility is less than zero,” Trump said. “I hope you’re going to remember that on Friday. When she tells you about the border, ask her just one simple question: “Why didn’t you do it four years ago?”

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That picks up on a theme Trump mentions at nearly all of his campaign rallies, scoffing at Harris as a former Biden administration “border czar,” arguing that she oversaw softer federal policies that allowed millions of people into the country illegally.

President Joe Biden tasked Harris with working to address the root causes of immigration patterns that have caused many people fleeing violence and drug gangs in Central America to head to the U.S. border and seek asylum, though she was not called border czar.

Since taking over for Biden at the top of the Democratic presidential ticket, Harris has leaned into her experience as a former attorney general of California, saying that she frequently visited the border and prosecuted drug- and people-smuggling gangs in that post. As she campaigns around the country, the vice president has also lamented the collapse of a bipartisan border security deal in Congress that most Republican lawmakers rejected at Trump’s behest.

Harris has worked to make immigration an issue that can help her win supporters, saying that Trump would rather play politics with the issue than seek solutions, while also promising more humane treatment of immigrants should she win the White House.

In June, Biden announced rules that bar migrants from being granted asylum when U.S. officials deem that the southern border is overwhelmed. Since then, arrests for illegal border crossings have fallen.

Despite that, a new poll by The Associated Press-NORC Center for Public Affairs Research released this month found that Trump has an advantage over Harris on whom voters trust to better handle immigration. This issue was a problem for Biden, as well: Illegal immigration and crossings at the U.S. border with Mexico have been a challenge during much of his administration. The poll also found that Republicans are more likely to care about immigration.

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Associated Press writer Will Weissert contributed to this report.