President-elect Trump, Democratic lawmakers and the Supreme Court are on a collision course over restricting birthright citizenship, which Trump has identified as a top priority for his incoming administration.

The U.S. is one of more than 30 countries with unrestricted birthright citizenship. Trump said in a recent interview that he plans to revoke the right.

In United States v. Wong Kim Ark, the U.S. Supreme Court ruled that the 14th Amendment provides people born in the U.S. citizenship at birth.

Donald Trump’s team is crafting an executive order to end birthright citizenship for the children of undocumented immigrants, a monumental move the president-elect’s allies say is a key step in their long-term strategy: getting the issue before the Supreme Court.