Moving to protect its constitutional power over the nation’s purse, the House of Representatives on Tuesday evening voted by a large majority to reject President Trump’s move to defy Congress’s refusal to provide extra money to pay for a U.S.-Mexico border wall that he intends to move ahead with on his own.

The vote was a unique political rebuke of a president, but the ultimate fate of the effort depends on how the Senate reacts, and also awaits a Trump veto if the measure should clear both houses of Congress.  Even so, the mere fact of the outcome in the House was a historic constitutional event.

The Constitution specifies that the federal government cannot spend any money unless Congress specifically approves the outlay, and it is President Trump’s plan to spend up to $8.1 billion for his border project.   Congress put up about $1.4 billion for border security but denied any of that for building a wall.  For the remainder of his project’s budget, the President plans to tap other funds previously voted by Congress, but none of that money was approved by Congress for a border wall.

The House vote marked the first time that either house of Congress had used authority that has existed for 43 years to undo a president’s claim that a national emergency justified an official government action.  Past presidents have declared such emergencies some 60 times in various situations, but not one of those met the fate that came in the House’s 254-to-182 vote to “terminate” the Trump emergency order.

If that same margin were repeated following a presidential veto, it would not be enough: it was 31 votes short of being veto-proof.  The vote for termination won the support of the 232 Democrats who took part; no Democrat voted against it.  Day-long efforts by Republican leaders to hold down the number in their ranks who would support the measure worked well, with only 13 members joining with the Democrats.

The Senate is scheduled to hold a vote on the termination measure before beginning a recess on March 18, and the vote in the Republican-controlled upper chamber is expected to be much closer than the margin in the Democratic-controlled House.

It would take a simple majority of 51 Senate votes to send the termination resolution to the White House.  If that happens, the President has vowed a veto and it would take the votes of two-thirds of each house of Congress to override the veto and accomplish the actual end of the emergency declaration and the President’s wall spending plan.

At this point, the most realistic prospect that the border wall project might actually be stopped would be a defeat of the plan in the federal courts.   Five major constitutional lawsuits have been filed – two in California, two in Washington, D.C., and one in Texas – challenging the presidential claim that there is an emergency over immigration across the southern border, and challenging the plan to spend money without explicit congressional approval.

Each of those lawsuits is in a very early stage, so there may not be a significant decision emerging from any of those courts until after Congress and President Trump have completed the steps now underway.

It is an open question whether the House vote, and the possibility of a similar outcome in the Senate, would have any influence on how the courts decide the pending constitutional challenges.

On the one hand, the prospect of a majority vote in both houses to reject the President’s project could be interpreted as a significant sign that the national emergency order was not justified under existing federal law or the Constitution, and as a sign that Congress had done all it could on its own to protect its basic legislative power and needed the support of the courts to sort out the constitutional division of power between the other two branches.

On the other hand, if President Trump in the end prevails, because the two houses of Congress cannot muster the votes to override a veto, that could be interpreted in court as an indication that the constitutional method of legislation had followed the proper course, thus settling the matter without the need for the courts to referee what might be seen as an inter-branch political conflict.