It is a task that comes with some political risks for Harris, a potential future presidential candidate.
But more of the migrants are children, officials say, and the border influx has provided Republicans a line of attack against Biden, who in his first weeks has succeeded in his promises to ramp up Covid vaccinations and to push through an economic package that offers relief to most Americans.
Biden halted construction on Trump's cherished wall on the Mexican border and has moved to wind down a controversial Trump policy, which migrant advocates say violates global conventions, in which asylum seekers are forced to wait in Mexico while their cases are heard.
Biden pushed back on Republican charges of a crisis and voiced compassion for people from Guatemala, Honduras and Nicaragua who are fleeing violence and poverty.
The surge in migration has created a hard problem for Ms Harris and Mr Biden, who both campaigned on promises to reverse Trump-era policies of holding unaccompanied minors and adults in "inhumane" cages.
"I can't guarantee we're going to solve everything, but I can guarantee we can make it everything better".
According to analysis from the Pew Research Center, there were nearly 19,000 family members counted in February this year, up by 168 percent from January.
A delegation of White House officials is also visiting a temporary HHS facility in Carrizo Springs, Texas, along with several members of Congress.
"He has experience and lived through this process as vice president himself", an official said. Images of children and teens crowded together inside border facilities have drawn national attention and heightened scrutiny of Biden's handling of the situation.
The legislator said one pod meant to hold 260 people was housing 400 boys. Are we looking at overcrowding at the border in particular of these kids? "Yes", Harris said during the Wednesday interview.
After that, migrant children are supposed to be transferred to a more hospitable facility run by the Department of Health and Human Services.
"Should we be processing these cases faster?" she asked. Harris's law enforcement background as the former attorney general of California was an important factor in the choice, the official said.
Harris said the current situation was in part caused by the Trump-era dismantling of a system that allowed USA officials to process children's asylum requests in their home countries.
"This increase has been consequential, but the vice president has agreed, among the multiple other things that I have her leading, and I appreciate it, agreed to lead our diplomatic effort and work with those nations to accept the returnees and enhance migration enforcement efforts at their borders", Biden said when he announced Harris' new role Wednesday.
Biden also said the United States would likely miss a May 1 deadline set by former President Donald Trump to pull all troops from Afghanistan, although he made clear he expected to end America's longest war within the year.
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