President Donald Trump issued a fresh veto threat as the Senate seeks to rebuke him over his emergency declaration at the US-Mexico border. Picture: Reuters/Jonathan Ernst

Washington - President Donald Trump issued a fresh veto threat Thursday morning as the Republican-led Senate appeared poised to pass a measure rebuking him over his national emergency declaration at the U.S.-Mexico border.

"A big National Emergency vote today by The United States Senate on Border Security & the Wall (which is already under major construction)," Trump said in an early-morning tweet. "I am prepared to veto, if necessary. The Southern Border is a National Security and Humanitarian Nightmare, but it can be easily fixed!"

Trump declared the national emergency on February 15 as a way to circumvent Congress and get the money he wants to build barriers along the southern border.

The president has framed the issue in terms of national security, whereas Democrats and some Republicans have focused on the constitutional issues at stake in Trump intruding on Congress's control over government spending.

On Wednesday, Sen. Mike Lee, R-Utah, who was leading an effort to craft a compromise aimed at curtailing presidential emergency powers in the future, became the latest Republican to announce plans to defect and vote for a resolution to nullify Trump's declaration when it comes to the floor on Thursday.

Lee made the announcement shortly after hearing directly from Trump that his legislation was not acceptable.

Lee's position all but ensures there will be majority support in the Senate on Thursday for the disapproval resolution, which has already passed the House. Senate passage would send the measure to Trump, forcing him to issue the first veto of his administration to strike it down.

The Washington Post