Trump said border security must be part of legislation to fund the U.S. government, a position that raises the threat of a partial federal shutdown on Saturday.
Trump told fellow Republican leaders in Congress he will not sign a Senate-passed bill because it fails to include enough funding for border security. Republicans in the House of Representatives were scrambling to try to amend the bill to appease Trump.
Trump has demanded $5 billion to put toward building a wall on the U.S. border with Mexico that he argues is needed to keep out illegal immigrants and drugs, a downpayment on a massive project which Democrats have rejected as ineffective and wasteful.
The latest plot twist in a long-running drama over government spending, occurring just as lawmakers pushed to take their Christmas break, emerged 36 hours before funding expires for agencies responsible for federal law enforcement activities, airport security screenings, space exploration and farm programs.
Trump was expected to start a two-week vacation in Florida on Friday.