The Trump administration announced Tuesday it would begin to unwind an Obama-era program that allows younger undocumented immigrants to live in the country without fear of deportation, calling the program unconstitutional but offering a partial delay to give Congress a chance to address the issue.

The decision, after weeks of intense deliberation between President Trump and his top advisers, represents a blow to hundreds of thousands of immigrants known as “dreamers” who have lived in the country illegally since they were children. But it also allows the White House to shift some of the pressure and burden of determining their future onto Congress, setting up a public fight over their legal status that is likely to be waged for months.