"We write as former United States Attorneys who have served under both Republican and Democratic Presidents", the letter begins.
Ms Nielsen was widely criticised on Sunday when she tweeted that her department does "not have a policy of separating families at the border. These images are eerily reminiscent of the Japanese American internment camps of World War II, now considered to have been one of the most shameful episodes in USA history".
The legislation was expected to take an approach similar to the family separation provisions in the House Republican bills. For those who come with families, their children are removed from them.
"Now as we watch with broken hearts, that's exactly what's happening", she said. "Rather, its implementation and its execution are taking place exclusively at your direction, and the unfolding tragedy falls squarely on your shoulders".
The Trump administration insists the family separations are required under the law.
President Trump doubled down on his administration's practice of separating illegal immigrant families at the Mexican border yesterday, saying the United States "will not be a migrant camp", as outrage mounted with MA officials blasting the policy and demanding immediate change.
Chris Sosa is the Senior Editor at AlterNet.
Mr Sessions was asked in Monday night's interview on Fox News about a tweet by former CIA Director Michael Hayden likening what happened at Auschwitz concentration camp, where millions of Jews and other minorities were killed, to the separation of undocumented immigrant families at the United States border.
U.S. Border Patrol agents ask a group of Central American asylum seekers to remove hair bands and wedding rings before taking them into custody on June 12, 2018 near McAllen, Tex.
The three co-founders of the room and house rental service said in a joint statement that "ripping children from the arms of their parents is heartless, cruel, immoral and counter to the American values of belonging".
Attorney General Jeff Sessions: "I would cite you to the Apostle Paul and his clear and wise command in Romans 13, to obey the laws of the government because God has ordained them for the goal of order".
Trump also falsely blamed Democrats for creating "crippling loopholes" that have resulted laws that force family separations at the border. The legislation wouldn't explicitly forbid the incarceration of children alongside their parents.
He said the practice was "deeply unethical" and that Republicans' refusal to criticise Mr Trump represented a "sad degeneration" of the GOP, which he said had become "the party of lies".
"Just remember - a country without borders is not a country at all". Senator Claire McCaskill of Missouri, one of the most vulnerable red-state Democrats up for re-election this year, said Sunday on Twitter that she would sign onto the bill this week and focus on the issue in her capacity as the top Democrat on the Homeland Security Committee.
"We are not that kind of country".
Amnesty International argues that these acts deliberately inflict such mental suffering that they constitute torture - inflicted not just on adults facing harsh choices, but also children who can not make choices at all.
The administration is under pressure to reverse an immigration enforcement policy that has led to the separation of almost 2,000 migrant children from their families in recent weeks.