WASHINGTON — Trying to turn the tables politically on President Trump’s own immigration policy, administration officials on Tuesday defended new measures they are taking to separate children from migrant parents who cross the United States’ southern border without authorization, blaming the situation on Democrats who have bitterly denounced it.

Top officials from the White House, the Justice Department and the Department of Homeland Security argued that a series of court rulings and laws that existed when Mr. Trump took office had essentially given them no choice but to carry out a policy that many human rights groups have condemned as inhumane.

The officials said it was Democrats, who have opposed Mr. Trump’s efforts to impose stricter immigration policies, who were forcing them to use the bare-knuckle tactics.

“A nation cannot have a principle that there will be no civil or criminal immigration enforcement for somebody traveling with a child,” Stephen Miller, Mr. Trump’s senior adviser for policy and an architect of the president’s immigration agenda, said during a conference call with journalists.