Ted Lieu, who represents California in the US House of Representatives - the lower chamber of Congress - attacked a new "zero-tolerance" approach announced by leaders.
Commenting on Twitter, he said: "There are many ways to describe the Donald Trump policy of ripping children away from their parents at the border.
"It violates human rights laws. It is un-American. It would shock Jesus. But I think the most appropriate way to describe it is this: the policy is evil."
Ted Lieu previously criticised more than 150 conservative Christian leaders in the US who signed the so-called Nashville Statement defending traditional teaching on sexuality and marriage.
The Gospel of Matthew states: "For I was hungry and you gave me food, I was thirsty and you gave me drink, I was a stranger and you welcomed me" Matthew 25:35.
— Ted Lieu (@tedlieu) May 26, 2018
The Bible does not say: "separate children from their parents and don't track the children."#WhereAreTheChildren https://t.co/ezGBEPc4zS
Earlier this month, it was revealed the Department of Homeland Security intends to ensure everyone caught crossing the border illegally would be referred for federal prosecution.
Attorney General Jeff Sessions told a meeting of the Association of State Criminal Investigative Agencies: "If you cross the border unlawfully, even a first offense, we're going to prosecute you.
"If you're smuggling a child, we're going to prosecute you, and that child will be separated from you, probably, as required by law.
"If you don't want your child to be separated, then don't bring them across the border illegally."
Critics fear a tougher approach could see immigrants given criminal records even if it a judge eventually decides that they have a valid asylum claim.
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.