A federal judge has struck down a Biden administration program that would allow unauthorized immigrants married to American citizens to get legal status and U.S. citizenship, declaring the policy illegal.

U.S. District Court Judge J. Campbell Barker, an appointee of President-elect Donald Trump, found the program violates U.S. immigration law, agreeing with a lawsuit filed by Texas and more than a dozen other Republican-led states.

The ruling is a major defeat for the outgoing Biden administration, which argued the policy, known as Keeping Families Together, promoted family unity among mixed-status households.

When it was announced earlier this year, officials said roughly half-a-million undocumented immigrants were likely eligible for the program.

The Justice Department can appeal Thursday’s ruling, but the incoming administration of Trump, has vowed to dismantle President Biden’s immigration policies. Trump has separately promised to seal and militarize the U.S.-Mexico border and oversee the largest mass deportation in American history.

The Keeping Families Together initiative was announced by Mr. Biden in June, just weeks after he took a different executive action to sharply limit asylum along the U.S.-Mexico border.

The program would give work permits and deportation protections to undocumented immigrants who are married to American citizens and have lived in the U.S. for at least 10 years without committing serious crimes.

Most importantly, the policy would also allow these immigrants to apply for permanent residency, also known as a green card. After three years, green card holders married to U.S. citizens can apply for citizenship.