President Trump said Wednesday the sheer amount of drugs flowing across the border — from meth to cocaine to deadly fentanyl — bolsters his case for declaring an emergency at the border, as Congress prepares to complete its rebuke of his decision to go around lawmakers for funding. “We’ve got to get the wall up, otherwise it all doesn’t work,” Mr. Trump said. The administration is building the case for his wall — a core 2016 campaign promise — ahead of a Senate vote Thursday to disapprove of the emergency Mr. Trump declared to tap Pentagon funds for the project. The House already passed a resolution to overturn the declaration, and enough Republicans appear ready to join Democrats in the Senate to approve it, though not enough to override Mr. Trump’s expected veto. “We’ll see whether or not I have to do the veto,” Mr. Trump said. The president also downplayed his decision’s impact on Republicans, who’ve been forced to choose between supporting the White House and defending Congress’ power over the nation’s purse strings. “Nobody’s beaten up. I said, ‘Use your own discretion,’” he said. The president would prefer to avoid the embarrassing rebuke, however. He said Republican senators are “overthinking” the vote and should see it as a simple choice to bolster border security and prevent crime. “This is a vote on border security and drugs and trafficking and all of that,” Mr. Trump said. “I think most Republican senators fully understand that.” Also Wednesday, Vice President Mike Pence visited a Customs and Border Protection training facility in West Virginia to build the case for the wall. “Despite the extraordinary work of all of you and those you represent in law enforcement, drug cartels and smugglers are exploiting this crisis to flood drugs into our country, tearing apart our families, claiming American lives,” he told officers.