Democratic lawmakers vowed Sunday to end the “evil” separation of migrant children from their parents at the US border, as First Lady Melania Trump made a rare political plea to end the deeply controversial practice.

The “zero-tolerance” border security policy implemented by President Donald Trump’s administration has sparked outrage on both sides of the political aisle and took on particular resonance as America celebrated Father’s Day.

“They call it ‘zero tolerance,’ but a better name for it is zero humanity, and there’s zero logic to this policy,” said Senator Jeff Merkley of Oregon, after leading a group of Democratic lawmakers to the Mexican border.

Trump’s wife, who seldom wades into the political arena, opted to call for bipartisan immigration reform to fix the issue.

“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,” her spokeswoman Stephanie Grisham told CNN.

“She believes we need to be a country that follows all laws, but also a country that governs with heart.”

The president himself later tweeted: “The Democrats should get together with their Republican counterparts and work something out on Border Security & Safety. Don’t wait until after the election because you are going to lose!” he tweeted.

Immigration is one of the most divisive issues plaguing the Trump administration.

The number of separations has jumped since early May, when Attorney General Jeff Sessions announced that all migrants illegally crossing the US border with Mexico would be arrested, regardless of whether the adults were seeking asylum.

Since children cannot be sent to the facilities where their parents are held, they are separated, which the American Academy of Pediatrics has warned causes “irreparable harm” to the children.

Homeland Security Secretary Kirstjen Nielsen insisted, however, that “we do not have a policy of separating families at the border. Period.”

“For those seeking asylum at ports of entry, we have continued the policy from previous Administrations and will only separate if the child is in danger, there is no custodial relationship between ‘family’ members, or if the adult has broken a law,” she wrote on Twitter.

Some of Trump’s fellow Republicans have said the policy must end.

“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,” Senator Susan Collins told CBS television’s “Face the Nation.”

“That’s traumatizing to the children who are innocent victims, and it is contrary to our values in this country.”

Laura Bush, the former first lady and wife of Republican ex-president George W. Bush, was unflinching in her rejection of the policy.

“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,” Bush, who lives in Texas, wrote in a Washington Post opinion piece.

Ex-president Bill Clinton, a Democrat, also weighed in.

“These children should not be a negotiating tool. And reuniting them with their families would reaffirm America’s belief in & support for all parents who love their children,” he tweeted.

In one incident that highlighted the crisis, five immigrants died and several others were injured after a high-speed chase with Border Patrol agents in Texas ended in a crash, US media said.

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