President Trump is pledging a veto if Congress passes a resolution blocking his use of a national emergency declaration to fund a wall along the U.S.-Mexico border.
Mr. Trump told reporters at the White House on Friday that he would vetoearlier in the day.
"Will I veto it? 100 percent," the president told reporters.
The move by House Democrats starts the clock on a constitutional clash between Mr. Trump and Democrats and sets up a vote by the full House as soon as next week.
The Democratic-controlled House is sure to pass the measure, although its fate in the GOP-controlled Senate is less clear.
Any Trump veto would likely be sustained, but the upcoming battle will test Republican support for Mr. Trump's move.
A handful of Republicans have voiced their strong opposition to Mr. Trump's decision to go around Congress.
Texas Republican Sen. John Cornyn said, "After the American Revolution against a king, our founders chose not to create a chief executive with the power to tax the people and spend their money any way he chooses. The Constitution gives that authority exclusively to a Congress elected by the people."
Sen. Chuck Grassley of Iowa, the chairman of the Senate Finance Committee, told Politico, "I wish he wouldn't have done it."
Meanwhile, the president's emergency declaration is facing an uphill battle in the courts.are suing Mr. Trump over the declaration.