A new law that gives local law enforcement in Texas the authority to arrest migrants “violates the US Constitution,” the Department of Justice said in a letter to Gov. Greg Abbott on Thursday, threatening to sue unless Abbott backs down from enforcing it.

The letter, obtained by CNN, marks the latest escalation between President Joe Biden and Abbott over the handling of the US-Mexico border.

Earlier this month, Abbott, a Republican, signed into law the measure, known as Senate Bill 4, which gives local law enforcement the power to arrest migrants and gives judges the ability to issue orders to remove people from the United States. The White House slammed the law, calling it “incredibly extreme.” It’s expected to take effect in March.