In a lengthy statement Monday morning, Sen. Ben Sasse called family separation "wicked" and "harmful to kids." The policy decision is a "new, discretionary choice" by the White House, the Nebraska Republican said.
"The president should immediately end this family separation policy," the senator said. He added that Trump should propose to Congress ways to resolve a legal settlement that requires the government to keep parents and children together for only a limited period of time.
Rep. Will Hurd, R-Texas, also called the policy "absolutely unacceptable." On Monday, he told NPR that "taking kids from their mothers is not preventing terrorists or drugs from coming into this country."
In addition, he contended that two House Republican immigration bills the chamber plans to consider this week would not resolve the problem.
"You don't need legislation. The administration can do this and stop this policy right now. But there's nothing that I've seen in this upcoming legislation that would stop this problem," he said.
Sen. Susan Collins, R-Maine, also called Sunday for the administration to "put an end" to the separation of children from parents who enter the country at a legal port of entry. Speaking to the CBS program "Face the Nation," she called it "inconsistent with our American values to separate these children from their parents unless there's evidence of abuse or another very good reason."
Sens. James Lankford, R-Okla., and Lindsey Graham, R-S.C., have also criticized the Trump administration policy.
Political pressure on the president has increased in recent days as advocates and lawmakers have flocked to buildings where children are housed. More images of the facilities have surfaced, with some taken in Texas showing children sleeping on thin mattresses on concrete floors within cage-like metal fencing.
Trump dug in on the policy on Monday morning. The president appears to be using the separation of families as leverage to secure votes for legislation to fund his proposed border wall and limit legal immigration.
In a tweet Monday morning, he again falsely blamed Democrats — who hold a minority in Congress — for the family separation crisis. He urged Congress to "change the laws!"
Homeland Security Secretary Kirstjen Nielsen also contended Monday that Congress needs to revise laws to end the policy. She even went as far as to claim Sunday that the White House did not have a policy of separating families. She contradicted other administration officials, including Sessions and policy advisor Stephen Miller, who have described the policy as a deterrent.
Among others who have criticized Trump's policy, former first lady Laura Bush called it "cruel" and "immoral." She added that it "breaks my heart."
Sen. Orrin Hatch, R-Utah, criticized the practice of splitting up families on Monday, but did not blame the Trump administration or put the burden on the president to end it. He said Congress should work on a "path forward that recognizes the need for compassion for children and families without incentivizing illegal border crossings."