Congress can veto Mr Trump's veto, but it does not look likely that they will have the necessary votes to do so.
Trump's veto sends the resolution back to the US House of Representatives, which is expected to pick it up after the week-long congressional recess. Unless more Republicans defect from the president's position, neither margin would be sufficient to override a veto. "This is not about the president or border security, in fact I support border security, I support a barrier".
The bill had passed the House in February with thirteen congressional Republicans crossing the aisle to join Democrats. "This will help stop Crime, Human Trafficking, and Drugs entering our Country", the president tweeted.
On 15 February Trump invoked the National Emergencies Act of 1976, claiming there was a crisis on the border that required the construction of walls to protect the United States.
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The emergency declaration is being challenged in court as an unconstitutional usurpation of Congress' power of the purse.
She also says the president "is incredibly disappointed" with Republicans who voted against him.
Trump insists he's on solid legal ground, however.
The renegade conservatives had condemned the emergency declaration for setting up a unsafe precedent for a president while emphasising that they still agreed with Mr Trump's tough border security policies. The measure ultimately passed 59-41, and Trump immediately vowed to veto. Mike Lee, R-Utah; Rob Portman, R-Ohio; Susan Collins, R-Maine; Lisa Murkowski, R-Alaska; Marco Rubio, R-Fla.; Rand Paul, R-Ky.; Lamar Alexander, R-Tenn.; Roger Wicker, R-Miss.; Roy Blunt, R-Mo.; Jerry Moran, R-Kan.; and Pat Toomey, R-Pa. The House previously passed the resolution last month, 245 (including 13 Republicans) to 182.
On Friday, Barr also said the president's emergency order was "clearly authorized under the law".