WASHINGTON, March 14 (Reuters) - The U.S. Senate passed a proposal on Thursday to terminate President Donald Trump's declaration of an emergency at the southern border, with 12 Republicans defying the president, and Trump vowing a veto.
The 59-41 vote marks the second Senate rebuke of Trump in two days. Senators on Wednesday approved a resolution seeking to end U.S. support for the Saudi Arabia-led coalition in the war in Yemen, rejecting Trump's policy toward the kingdom.
During the first two years of his term, the Republican-led Congress mostly accommodated Trump, who has not yet used his veto pen.
With the emergency declaration, Trump was seeking an alternative way to get billions of dollars for the wall after Congress declined to give him funding.
Senate Majority Leader Mitch McConnell had urged his fellow Republicans to defeat the measure, which was passed in February by Democratic-controlled U.S. House of Representatives.
Trump had tweeted on Thursday that a vote for the resolution by Republican senators would be vote for Democratic House Speaker Nancy Pelosi as well as "Crime and the Open Border Democrats!"
Republicans who defected by supporting the measure to end the emergency declaration are worried that presidents - including future Democratic ones - could usurp the power of Congress to fund the government and use the tactic to pass their own pet programs.
McConnell, however, said Trump was "operating within existing law" and that if senators did not like the powers provided to the president under the National Emergencies Act, "then they should amend it."
The measure is unlikely to become law since a two-thirds vote of Congress is needed to override the presidential veto that Trump has vowed to issue. The dispute could ultimately be decided by the courts.
At stake are billions of dollars in funding for a wall along the U.S.-Mexico border that Trump is demanding but Congress has refused to fully provide. The stalemate led to a 35-day partial government shutdown that ended in January.
Trump made the wall a keystone of his 2016 presidential campaign and said Mexico would pay for it.
Under the emergency declaration Trump signed on Feb. 15, he would take money from other federal programs to build the barrier, which he says is needed to curb illegal immigration and drug trafficking.
Even with a veto threat looming, senators and legal experts said Congress was sending an important message that could be cited by judges in several lawsuits that have been filed challenging Trump's emergency declaration.
"It's an important legal statement," said Senator Angus King, one of two Senate independents. "It tells the court this is explicitly not approved by Congress By voting this resolution Congress is reiterating we don't approve of this expenditure."
Peter Shane, a law professor at the Moritz College of Law at Ohio State University, agreed.
"Congress's vote, even if vetoed, would solidify any court's understanding that the so-called emergency is really part of an end-run around a legislative branch that is unconvinced an emergency exists, that refused to fund the wall, and that is constitutionally in charge of federal spending," he said.
Vice President Mike Pence met with Republican senators this week to try to tamp down support for terminating the declaration. He said Trump would back a second bill offered by Republican Senator Mike Lee that would end future emergency declarations after 30 days unless Congress votes to extend them. Lee said on Wednesday the White House had subsequently made clear his bill did "not have an immediate path forward."
Democrats deny there is an emergency at the border, saying border crossings are at a four-decade low.
"Democrats and Republicans both know the sad truth: the president did not declare an emergency because there is one," Senate Democratic Leader Chuck Schumer said on the Senate floor. "He declared an emergency because he lost in Congress and wants to get around it."
King expressed frustration that the Department of Defense has so far refused to provide Congress with the projects it would have to suspend under Trump's emergency declaration in order to fund the wall.
"We have a reasonable expectation of knowing what the universe of potential projects cuts are before we take this vote," King said in a telephone interview with Reuters shortly before the vote, adding that Trump is attempting to "undo" $3.6 billion worth of defense projects already approved by Congress. (Reporting by Amanda Becker and Richard Cowan; additional reporting by Susan Cornwell, Doina Chiacu, Lawrence Hurley and Susan Heavey; editing by Jeffrey Benkoe and Bill Trott)