The emotional policy of separating children from their parents is also starting to divide Republicans and their allies as Democrats turn up the pressure.
while GOP Sen. Susan Collins expressed concern about it and a former adviser to President Donald Trump said he thought the issue was going to hurt the president at some point. Religious groups, including some conservative ones, are protesting.
Mrs. Bush made some of the strongest comments yet about the policy from the Republican side of the aisle.
"I live in a border state. I appreciate the need to enforce and protect our international boundaries, but this zero-tolerance policy is cruel. It is immoral. And it breaks my heart," she wrote in a guest column for the Washington Post Sunday. She compared it to the internment of Japanese-Americans during World War II, which she called "one of the most shameful episodes in U.S. history."
Republican Sen. Susan Collins of Maine said she favors tighter border security, but expressed deep concerns about the child separation policy.
"What the administration has decided to do is to separate children from their parents to try to send a message that if you cross the border with children, your children are going to be ripped away from you," she said. "That's traumatizing to the children who are innocent victims, and it is contrary to our values in this country."
Former Trump adviser Anthony Scaramucci said in a weekend interview that the child separation interview could be dangerous for Trump. He said the president "should be immediately fixing this problem."
"This is a fuse that has been lit," he said. "The president is going to get hurt by this issue if it stays out there very, very long."
The signs of splintering of GOP support come after longtime Trump ally, the Rev. Franklin Graham, called the policy "disgraceful." Numerous religious groups, including some conservative ones, have pushed to stop the practice of separating immigrant children from their parents.
This pressure is coming as White House officials have tried to distance themselves from the policy. Trump blames Democrats falsely for the situation. The administration put the policy in place and could easily end it after it has led to a spike in cases of split and distraught families.
"Nobody likes" breaking up families and "seeing babies ripped from their mothers' arms," said presidential counselor Kellyanne Conway.
Nearly 2,000 children were separated from their families over a six-week period in April and May after Attorney General Jeff Sessions announced a new "zero-tolerance" policy that refers all cases of illegal entry for criminal prosecution. U.S. protocol prohibits detaining children with their parents because the children are not charged with a crime and the parents are.
Trump plans to meet with House Republicans on Tuesday to discuss pending immigration legislation amid an election-season debate over one of his favorite issues. The House is expected to vote this week on a bill pushed by conservatives that may not have enough support to pass, and a compromise measure with key proposals supported by the president. The White House has said Trump would sign either of those.
Conway rejected the idea that Trump was using the kids as leverage to force Democrats to negotiate on immigration and his long-promised border wall, even after Trump tweeted Saturday: "Democrats can fix their forced family breakup at the Border by working with Republicans on new legislation, for a change!"
Asked whether the president was willing to end the policy, she said: "The president is ready to get meaningful immigration reform across the board."
To Rep. Adam Schiff, D-Calif., the administration is "using the grief, the tears, the pain of these kids as mortar to build our wall. And it's an effort to extort a bill to their liking in the Congress."
Schiff said the practice was "deeply unethical" and that Republicans' refusal to criticize Trump represented a "sad degeneration" of the GOP, which he said had become "the party of lies."
"There are other ways to negotiate between Republicans and Democrats. Using children, young children, as political foils is abhorrent," said Sen Jack Reed, D-R.I.
Even first lady Melania Trump, who has tended to stay out of contentious policy debates, waded into the emotional issue. Her spokeswoman says that Mrs. Trump believes "we need to be a country that follows all laws," but also one "that governs with heart."
"Mrs. Trump hates to see children separated from their families and hopes both sides of the aisle can finally come together to achieve successful immigration reform," spokeswoman Stephanie Grisham said.