Five Republican senators have said they back the measure passed in February by the US House of Representatives, which is controlled by Democrats. At least four Republicans are needed to pass it in the 100-seat Senate, along with all 45 Democrats and two independents.
But the measure is unlikely to become law given that a two-thirds vote of Congress is needed to override a presidential veto, which Trump vowed to issue if it passed the chamber Thursday.
“I am prepared to veto, if necessary,” Trump said in an early-morning post on Twitter before senators took up the measure, Reuters reported.
Vice President Mike Pence met with Republican senators this week to try to tamp down support for the measure, with some Republicans worried that future Democratic presidents could usurp the power of Congress to fund the government and use the emergency declarations to fund their own pet programs.
Pence told senators that Trump would back a second bill offered by Republican Senator Mike Lee, which 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.” He added he would vote on Thursday to end the emergency declaration.
At stake are billions of dollars in funding for a wall along the US-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.
Under the emergency declaration Trump signed on Feb. 15, he would take money from other federal programs to build the barrier he says is needed to curb illegal immigration and drug trafficking.
Democrats deny there is an emergency at the border, saying border crossings are at a four-decade low.
Court challenges have also been filed asserting it is Congress, not the president that decides how taxpayer money is spent.