The Trump administration consistently said they had no choice but to separate families because of the law. "When you prosecute the parents for coming in illegally, which should happen, you have to take the children away", Trump said.
"So we will be officially postponing the congressional picnic for tomorrow".
May said images of migrant children kept in cage-like units were "deeply disturbing", and the Council of Europe, a global human rights watchdog, said Trump had abdicated any claim to moral leadership in the world.
Family separation is now a huge issue in the United States, where children have been taken away from their parents as a outcome of the Trump government's "zero tolerance" policy on illegal immigration.
The plan would keep families together in federal custody while awaiting prosecution for illegal border crossings, potentially violating a 1997 court settlement limiting the duration of child detentions.
House Democrat Juan Vargas participated in a loud and rare protest against a sitting president by US lawmakers inside the US Capitol.
"We need you, those children need you -and I am talking directly to my Republican colleagues- we need you to stand up to President Donald Trump", he said.
If the president goes through with the signing, "It would be a complete 180".
In a conference call with reporters Wednesday evening about the order, Gene Hamilton, counselor to Sessions, referred questions about families that have already been separated to DHS and the Department of Health and Human Services. But after signalling Monday that it would oppose any fix aimed exclusively at addressing that issue, the White House said Tuesday it was reviewing the emergency legislation being introduced by Cruz to keep migrant families together.
Under previous presidents, families of immigrants crossing the border without official paperwork tended to be issued with summonses for court, rather than be detained and split up.
"Right now we're focused on getting this bill passed", he said. "I offered the amendment to keep families together but also to have $7 billion for new housing for these family units", Denham said.
It also suggests the government intends to hold the families indefinitely by challenging an existing statute, the 1997 Flores Settlement, that places a 20-day limit on how long children, alone or with their parents, can be detained.
"Some of the members wanted to make sure the president is very visible in his support for both bills", said Rep. Mark Walker, R-N.C., the chairman of the conservative Republican Study Committee, which makes up more than half the GOP majority. But a new information-sharing agreement allows Homeland Security to obtain personal information on all potential sponsors, including their immigration status, a change that advocates say will have a chilling effect that discourages those living in the country illegally from picking up the children.
Even if Republicans manage to pass an immigration bill through the House, which is a tall order, the fight is all but certain to fizzle in the Senate.
Under the order, the zero-tolerance policy will remain in effect. The experts cite research showing that family separation can cause long-term trauma for children. "It's a problem that's gone on for many years, as you know, through many administrations".
The Flores settlement, named for a teenage girl who brought the case in the 1980s, requires the government to release children from custody and to their parents, adult relatives or other caretakers, in order of preference.
ICE operates two large family detention centers in Texas and a smaller facility in Pennsylvania, with a combined capacity for about 3,000 beds.
"He mentioned that his daughter Ivanka had encouraged him to end this and he said he does recognize that it needs to end and the images are painful and he's looking for a legislative solution", CNN reported Florida congressman Carlos Curbelo as saying.
"Neither party should want to separate kids from their parents", California Rep. Jeff Denham, another endangered Republican, said on CNN hours later.
Both bills include language to end the controversial White House policy of separating migrant parents from their children at the border. Last fall, the president nixed an Obama-era program that provided legal status and work permits to the so-called "Dreamers", the undocumented immigrants who came to the U.S.as children.