The Republican president in February declared a national emergency at the border to secure the money that Congress refused to give him for the wall.
“The President’s action clearly violates the Appropriations Clause by stealing from appropriated funds, an action that was not authorized by constitutional or statutory authority,” Pelosi, the top Democrat in Congress, said in a statement.
She said a congressional legal advisory group voted to authorize the lawsuit.
In a stinging rebuke, the Democratic-controlled House passed legislation to terminate the emergency Trump declared at the U.S.-Mexico border but Trump issued the first veto of his presidency in March to block that measure.
“The House will once again defend our Democracy and our Constitution, this time in the courts,” Pelosi said in a statement. “No one is above the law or the Constitution, not even the President.”
U.S. Attorney General William Barr said the president’s emergency declaration was legal.
(Reporting by Doina Chiacu; Editing by Chizu Nomiyama and James Dalgleish)