President Donald Trump’s administration is suing four states over their refusal to issue undercover license plates to federal agents, the latest front in the wider struggle between the White House and Democratic-led states over the Republican president’s immigration crackdown.

The Department of Justice alleges in separate lawsuits filed Wednesday that Maine, Massachusetts, Oregon, and Washington state are imposing unconstitutional restrictions that it says impede law enforcement and threaten agents’ safety.

“By denying undercover license plates to DHS components, including ICE, while issuing them to their own state agencies, these governors are pursuing discriminatory and obstructionist policies against federal law enforcement,” said acting Attorney General Todd Blanche in a statement.

“These actions undermine federal immigration enforcement, allow dangerous criminals to escape justice, and terrorize American communities,” Blanche added.

The Justice Department filed individual suits in U.S. district courts in the respective states. The four state governments are accused of trying “to obstruct the Federal Government’s immigration enforcement efforts, even though control over immigration and the nation’s borders is an exclusive federal power.”

Additionally, the Justice Department argues in the suits that the U.S. Constitution’s Supremacy Clause bars state governments from regulating federal law enforcement.