The federal lawsuit Wisconsin and 16 other states have filed, accuse President Obama of exceeding his authority and violating the Constitution.
They're referring to the executive action he exercised last month, allowing five million people living in the U.S. illegally, to avoid deportation.
Gov. Scott Walker asked Wisconsin Att. Gen. J.B. Van Hollen to join the multi-state challenge of the President's action, and Van Hollen agreed.
"It is clear that the President has exceeded his authority and that this important matter should be reviewed by the courts," Van Hollen said.
Walker released a statement reading, in part: "The immigration system is broken, but this is an issue that should be addressed through collaborative federal action, not unilateral action by the President."
The lead plaintiff in the federal suit is the state of Texas. Most of those signing on are located in the South and Midwest.