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By Adam Edelman

Thirteen Democratic lawmakers are calling on Homeland Security Secretary Kirstjen Nielsen to resign as opposition mounted on Tuesday, including from the U.S. Chamber of Commerce, to the Trump administration's policy of separating children from their parents at border.

Sen. Kamala Harris, D-Calif., condemned Nielsen's department for "tearing" families apart.

"The government should be in the business of keeping families together, not tearing them apart. And the government should have a commitment to transparency and accountability. Under Secretary Nielsen's tenure, the Department of Homeland Security has a track record of neither," Harris said in a statement on Monday. "As a result, she must resign."

House Minority Leader Nancy Pelosi, D-Calif., also called for Nielsen's resignation after touring a facility near San Diego that the federal government is using to house detained children who have been separated from the parents at the border under President Donald Trump's "zero tolerance" immigration policy.

"We have zero tolerance for your neglect and for your policy of selecting — of separating children from their parents," Pelosi said.

Other Democratic lawmakers calling for Nielsen's resignation included Sens. Richard Blumenthal of Connecticut, Mazie Hirono of Hawaii, Jeff Merkley of Oregon and Tina Smith of Minnesota, as well as Reps. Barbara Lee, Ted Lieu, Jimmy Gomez and Nanette Diaz Barragan, all of California; Kathleen Rice and Eliot Engel, both of New York; and Donald Payne of New Jersey.

The Department of Homeland Security, responded to lawmakers' calls in a statement, doubled down on Nielsen's claim that Congress should find a solution.

"Instead of criticizing a government official who is actually doing the job she was nominated, confirmed and sworn to do and enforcing the laws passed by Congress, the obstructionists in Congress should get to work to secure our borders, end legal loopholes and protect American lives," Tyler Houlton, the agency's press secretary, said.

A growing number of Republicans have called on the administration to end its policy of separating children from their parents, although they have stopped short of demanding Nielsen's resignation.

"I respectfully ask that you move immediately to rescind the policy that General Sessions announced in April and any other policies that have led to an increase in family separations at the border," Joe Straus, the Republican speaker of the Texas state House, wrote in a letter to Trump on Tuesday.

"None of us want to imagine the fear that overcomes young children who are forced out of their parents' arms, which adds to the trauma that they have often faced in their countries of origin," he added. "It is wrong to use the these scared, vulnerable children as a negotiating tool."

In calling for an end to the policy, the CEO of the U.S. Chamber of Commerce, Thomas Donohue, said Tuesday in a statement: "There is no other way to say it: This is not who we are and it must end now."

"Surely a nation as big, generous, and compassionate as the United States can find a way to prevent separating children from their parents at the border," Donohue said. "If we can’t agree on that, then we can’t agree on anything."

All four living former first ladies — Rosalynn Carter, Hillary Clinton, Laura Bush and Michelle Obama — also have condemned the Trump administration's practice of separating parents and children at the border.

The mounting calls for Nielsen's ouster came after she refused on Monday to apologize for separating children from their parents and even blamed Congress for the policy in two appearances on Monday — first at the National Sheriffs' Association annual conference and then at a White House press briefing.

"There has been much outcry, consternation and frankly misinformation for many in the press … that we at DHS are intentionally doing things that are unhumanitarian, cruel, immoral and disgraceful. We are doing none of those things," Nielsen said at the conference. "We are enforcing the laws passed by Congress, and we are doing all that we can in the executive branch to protect our communities. It is now time that Congress acts to fix our broken immigration system."

She continued: "This administration has a simple message: If you cross the border illegally, we will prosecute you. If you make a false immigration claim, we will prosecute you. If you smuggle illegal aliens across an extraordinarily dangerous journey, we will prosecute you.'

The outcry over Nielsen's response to the situation grew Monday after an audio recording emerged featuring the heart-wrenching cries of children inside a border processing center where the Trump administration is separating children from parents and guardians.

Young children wail and cry out "Papá" and "Mami" in the audio, which was first reported by ProPublica from inside a U.S. Customs and Border Protection facility; a copy was provided to NBC News by civil rights attorney Jennifer Harbury. The recording has not been independently verified by NBC News.

On Tuesday morning, over 70 former U.S. attorneys signed a letter to Attorney General Jeff Sessions condemning the breaking up of migrant families. "Traumatizing children by separating them from their parents as a deterrent for adult conduct is, in our view, sufficient reason to halt your policy," they wrote.

Also on Tuesday, 21 Democratic attorneys general, lead by New Mexico's Hector Balderas, sent a letter to Sessions and Nielsen urging the Trump administration to end its "zero tolerance policy."

"The Justice Department is ignoring its legal and moral obligations for the sake of a political agenda at the expense of children and the efforts of state law enforcement officials combating crimes like human trafficking, drug trafficking and gang violence which operate across international borders," Balderas said.

Balderas was joined by the attorneys general of California, Connecticut, Delaware, District of Columbia, Hawaii, Illinois, Iowa, Maine, Maryland, Massachusetts, Minnesota, New Jersey, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia and Washington.

Julia Ainsley contributed.