"Ever since Donald Trump was elected president what we have seen are Democrats obstructing him every step along the way that they can do it, even to the expense of our own nation's security", said Barrasso.
As president, Trump has the constitutional authority under Article I, Section 7 to reject a proposal from Congress. It is unlikely that Congress will have the two-thirds majority required to override Trump's veto, though House Democrats will try nonetheless.
The declaration is already the subject of a legal challenge.
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.
"House Republicans will have to choose between their partisan hypocrisy and their sacred oath to support and defend the Constitution", she added.
"Congress has the freedom to pass this resolution and I have the duty to veto it", he said, calling it a "Democrat-sponsored resolution".
Congressional critics do not appear to have the votes to override Trump's veto.
Alexander added that he shares the president's goal of border security, but warned allowing Trump to redirect money without authorization could set a unsafe precedent.
President Trump will deliver remarks in the Oval Office at 3:30 p.m. ET, White House says. Mr. Trump wasted no time countering, tweeting soon after, "VETO!"
"We clearly have a [border] crisis and we have to address it", Republican Senator Rob Portman told reporters. "The same principle should apply, regardless of which party occupies the White House".
Trump declared the national emergency after signing a government funding agreement that did not include the money he requested to build a wall.
Every one of the 12 Republican Senators who voted for a resolution opposing President Donald Trump's emergency order for the border is committed and united to securing the border, despite their vote, Sen.
"I think actually a national emergency was designed for a specific objective like this, so we have a great case", Trump said. "The way to do it constitutionally is to let the president declare an emergency, allow it to run 30 days, unless you have a vote of approval".