Rebel members of President Donald Trump’s party have helped pass a vote to reject his declaration of an emergency on the US-Mexico border.
Twelve Republican Senators broke party ranks to side with Democrats, approving a proposal to revoke the proclamation by 59-41.
The Democratic-controlled House of Representatives last month backed the measure.
Following Thursday’s vote, Mr Trump tweeted: “VETO!”
Congress needs a two-thirds majority of both chambers to override a presidential veto, which is viewed as unlikely.
Nevertheless, the defeat will be seen as an embarrassing loss for the president on his signature domestic issue.
It comes just a day after the Senate rebuked him on foreign policy by approving a bill to end US support for the Saudi-led coalition in Yemen.
Earlier on Thursday Mr Trump called Democrats “border deniers” and said any Republican opposing him would be casting “a vote for Nancy Pelosi”.
