First, Democratic Leader Chuck Schumer of New York announced Tuesday that he will force a vote to reject the plan, saying the vote would give lawmakers a chance to block Trump "from stealing military funding from their states to foot the bill for an expensive and ineffective wall he promised Mexico would pay for."
A similar measure passed this spring with 12 Senate Republican votes but was vetoed by Trump. The rules allow Schumer to retry every few months and don't allow Republicans to block the vote.
Perhaps more ominously for Trump is a potential vote on Thursday in the powerful Appropriations panel, where several members agree with Democrats that Trump is overstepping by reordering spending decisions by Congress to fund wall projects that have otherwise been rejected.
A top Appropriations Democrat, Sen. Dick Durbin of Illinois, said he will move to amend a $694 billion Pentagon funding bill to block Trump from diverting money intended for military projects to the wall.
The Pentagon last week identified $3.6 billion worth of military construction projects it's willing to kill to build 175 miles (282 kilometers) of border wall. The projects included a $63 million middle school in GOP Majority Leader Mitch McConnell's state of Kentucky though most of them are located outside the continental U.S.
"The cancellation of these projects is based on a national emergency declared by the president that was rejected on its face by both house of Congress on bipartisan votes," Durbin said. "Congress cannot and should not be silent when the power of the purse is undermined in this way. Why are we here?"
Durbin would prevail in the vote if panel Republicans like Roy Blunt of Missouri and Susan Collins of Maine vote like they did in March — a development that would embarrass top Republicans like McConnell and Appropriations Committee Chairman Richard Shelby.
McConnell said Durbin's move, along with an abortion-related vote promised by Sen. Patty Murray, D-Wash., on a separate health spending bill, amount to "poison pills" that violate the recent budget pact.
"I can't overstate how difficult it was, given the players involved, to reach an agreement," McConnell said. "And here, when we begin to take the first step in our committee process here, our Democratic friends ... are trying to wiggle out of the agreement we all signed off on and voted for. So I think it is truly a disturbing development."
In remarks caught on a live microphone after a panel vote on the whopping defense measure Durbin implored Shelby to work with him to stop Trump's border wall maneuvering. Shelby was clearly sympathetic.
"I'm going to do everything I can," Shelby said. "Listen, I'm going to talk to McConnell, and you talk to Schumer and let's see if we can get together."
A Shelby spokeswoman said the Alabama Republican was talking more generally about moving the appropriations process — which would fill in the details of this summer's bipartisan spending and debt deal — forward. Shelby is also pushing ahead, for now, to deliver Trump's full $5 billion request for new wall funding in the upcoming round of spending bills, though it's a non-starter with Senate Democrats and powerful House Speaker Nancy Pelosi, D-Calif.
Schumer said Trump is actually trying to win $12 billion for the wall once the $5 billion request is combined with money that Trump is shifting from military accounts.
"They know darn well that $12 billion additional for the wall is not going to fly — with Senate Democrats or with the House. So they ought to get to serious negotiating now that they've shown the president they're paying a little more fealty to him," Schumer said.
Schumer's move in the full Senate to force a repeat vote could put some Republicans in a difficult spot. For instance, endangered Republicans Cory Gardner of Colorado and Martha McSally of Arizona supported Trump in the earlier vote in March but stand to each lose funding for a home state project.
Trump says a wall would stop immigrants from entering the United States illegally. He promised repeatedly during his 2016 presidential campaign that Mexico would pay for the wall, but Mexico refused.