A federal judge on Thursday struck down a Biden administration program that would allow unauthorized immigrants married to American citizens to get legal status and a streamlined path to 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 Keeping Families Together program is likely to be in the crosshairs of the incoming administration of Trump, who has vowed to dismantle President Biden’s immigration policies. Trump has separately vowed to seal and militarize the U.S.-Mexico border and oversee the largest mass deportation in American history.

Representatives for the White House and Department of Homeland Security did not immediately respond to requests for comment.

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.

Immigrants who marry U.S. citizens are already eligible for a green card on paper. American immigration law, however, requires those who entered the U.S. illegally to leave the country and re-enter legally to be eligible for a green card. But leaving the U.S. after living in the country unlawfully for some time can trigger a 3 or 10-year exile, prompting many families to refrain from pursuing that option.

The Biden administration’s program would allow eligible immigrants to apply for a green card without having to leave the county by granting them an immigration benefit known as parole, which effectively cancels out their illegal entry.

In his ruling, Campbell Barker said the Biden administration did not have the legal authority to grant parole to unauthorized immigrants who are already in the U.S.

Camilo Montoya-Galvez

Camilo Montoya-Galvez is the immigration reporter at CBS News. Based in Washington, he covers immigration policy and politics.