(CNN) — As a border crisis unfolded only months into Joe Biden’s presidency, he looked to his vice president to help solve an intractable issue: migration.

It seemed like a no-win political assignment. Vice President Kamala Harris and her staff quickly sought to make one thing clear: She wasn’t charged with managing the southern border.

Three years later, that task looms over Harris’ unprecedented campaign for the White House, becoming a central line of attack from Republicans.

“As a result of her dangerously extreme immigration policies, the largest invasion in history is now taking place at our southern border, and it’s getting worse, not better,” former President Donald Trump said in a rare call with reporters Tuesday, falsely claiming Harris had been appointed Biden’s “border czar.”

The management of the US-Mexico border has been a political liability for Biden, and it will now follow Harris as Trump makes it a cornerstone of his campaign.

Over the last three years, an unprecedented number of border crossings have come to define the administration’s immigration record — recently resulting in the White House taking an aggressive measure to dramatically clamp down on asylum at the US southern border.

As the vice president’s campaign takes shape and as immigration remains a top issue for voters, her team is forced to contend with an assignment that, sources say, has showed early success in Central America as a result of major private-sector investment but that’s been bundled with the administration’s larger migration issues.

In the first rally of her 2024 presidential campaign Tuesday, Harris didn’t mention border security. The issue has generally not been featured prominently at campaign rallies over the last year, but both Harris and Biden have recently cited the bipartisan immigration deal that was scuttled by Trump to make the case that Republicans aren’t serious about border security.

The House GOP campaign arm is also encouraging lawmakers to focus on what it describes as Harris’ failed border policies, according to a memo obtained by CNN.

In a statement, Kevin Munoz, a spokesperson for Harris’ presidential campaign, cast Trump’s immigration policies as “xenophobic” and blamed Trump for dysfunction at the border.

“The only ‘plan’ Donald Trump has to secure our border is ripping mothers from their children and a few xenophobic placards at the Republican National Convention,” Munoz said. “He tanked the toughest bipartisan border security deal in a generation because for Donald Trump, this has never been about actually securing the border – it’s always about himself. He can make up whatever lies he wants but the fact is there’s only one candidate in this race who will fight for real solutions to help secure our nation’s border, and that’s Vice President Harris.”

Harris’ root cause work dates to March 2021. During an influx of unaccompanied migrant children, Biden tasked the vice president with overseeing diplomatic efforts in Central America, seeing the assignment as a sign of respect, having done the same job himself under former President Barack Obama.

While Harris focused on long-term fixes, the Department of Homeland Security remained responsible for overseeing border security.

At the time, most minors apprehended at the US southern border were from El Salvador, Guatemala and Honduras — a region that was hard-hit by major hurricanes and the Covid-19 pandemic and that had been a main source of migration over the last decade.

As the vice president’s team began strategizing, the problem facing the administration grew. Seven months later, it was migrants arriving from even farther away in South America — outside of Harris’ assigned portfolio — who were overwhelming the Biden administration.

Border crossings surged, and Republicans pointed their fingers at the vice president, dubbing Harris the “border czar,” a title the White House rejected, arguing that her focus was on the region and not on border security. And in 2022, as an affront to Harris, Texas Gov. Greg Abbott, a Republican, began busing migrants to her residence at the Naval Observatory in Washington, DC.

Democratic Rep. Pete Aguilar of California on Tuesday called efforts by House Republicans to paint Harris as a border czar “laughable and unserious.”

“Let’s be very clear, there was no ‘border czar.’ Kamala Harris’ role was to engage in multilateral discussion with our Latin American countries,” the House Democratic Caucus chair told reporters.

Harris has only occasionally talked about her efforts as the situation along the US-Mexico border became a political vulnerability for Biden.

And it’s the comments she made early on that will likely be used by Republicans in the months to come.

In an interview with NBC’s Lester Holt in June 2021, roughly five months after taking office, Harris was pressed about the fact that she hadn’t yet visited the US-Mexico border.

“At some point, you know, we are going to the border,” the vice president said. “We’ve been to the border. So this whole … thing about the border. We’ve been to the border. We’ve been to the border.”

Holt responded: “You haven’t been to the border.”

“And I haven’t been to Europe. And I mean, I don’t — I don’t understand the point that you’re making,” Harris said with a laugh. She added: “I’m not discounting the importance of the border.”

Later that month, Harris visited the border.

Harris navigates one of the most politically fraught issues

White House officials quickly worked to get Harris up to speed after she was assigned to tackle the root causes of migration.

Officials pulled together a series of memos examining what Biden had done when he was vice president and held a similar role, analyzing what worked and what could be done better, according to a former senior administration official.

In some ways, the region had already been familiar territory for Harris, dating to her days as California’s attorney general.

“In terms of the root causes piece as VP, there’s a through line in her work as AG starting to build relationships in Mexico and Central America,” said Daniel Suvor, former chief of policy to then-Attorney General Harris and now partner at O’Melveny & Myers, arguing that her work has often focused on concrete, measurable outcomes.

Officials met daily, briefing the vice president along the way, with another former senior administration official describing the process as “intense” amid a worsening crisis at the border.

“She ultimately studied that and put her own stamp on it, which was the private-sector component,” one of the former senior administration officials said, noting that was an area they concluded Biden, as vice president, didn’t have time to fully develop.

Harris pulled together the Partnership for Central America, which has acted as a liaison between companies and the US government. Her team and the partnership are closely coordinating on initiatives that have led to job creation in the region. Harris has also engaged directly with foreign leaders in the region.

Earlier this year, Harris met with President Bernardo Arévalo of Guatemala to strengthen the US-Guatemala bilateral relationship and discuss good governance, economic opportunity, security and migration management, according to a White House readout.

Around 56 companies are collaborating across financial services, textiles and apparel, agriculture, technology, telecommunications, and nonprofit sectors to bolster the region’s economy. Together, they’ve invested more than $5 billion.

Experts credit Harris’ ability to secure private-sector investments as her most visible action in the region to date but have cautioned about the long-term durability of those investments.

Honduran Minister of Investment Miguel Medina argued that having the White House behind the initiative has been instrumental in bringing big companies and private-sector money into the region.

“The difference with the partnership is that the facilitating that they’ve been doing, and they continue to do, that facilitating is not something that is accessible to a normal company in Honduras,” he said, citing, for example, work with Nespresso to purchase and sell coffee beans.

“If it wasn’t for this being moved from the White House, there’s … no way we could have had the success that it’s had,” Medina added.

While it’s difficult to measure the direct impact on migration, US Customs and Border Protection has seen a considerable drop in migrants arriving at the southern border from Guatemala, Honduras and El Salvador, according to federal data.

“For the long term, she certainly can claim credit for having started off efforts to improve people’s lives in Central America,” said Andrew Selee, president of the Migration Policy Institute, a nonpartisan think tank.

This story has been updated with additional reaction.

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