Sen. Thom Tillis (R-N.C.) on Capitol Hill on March 14. (Michael Reynolds/EPA-EFE/Shutterstock)

REGARDING PRESIDENT TRUMP’S pet project, a wall on the U.S.-Mexico border, the will of Congress is clear. For fiscal 2019, the executive branch may spend $1.375 billion to build a fence-like barrier in one sector of the region. That’s it — and Mr. Trump signed the bill on Feb. 15 to avert yet another government shutdown. What is more, a clear majority of Congress — 245 of 432 sitting representatives and, as we learned Thursday, 59 of 100 senators — has voted to terminate the national emergency Mr. Trump declared so he could devote $3.6 billion of other appropriated funds to a wall.

And yet Mr. Trump can still get his way, by vetoing that measure on the grounds that it was “a vote for Nancy Pelosi, Crime, and the Open Border Democrats!” as he tweeted. A vote for crime! This defeat for legitimate congressional prerogatives would be in part an unintended consequence of a 1983 Supreme Court ruling that made it impossible for Congress to overturn a national emergency declaration without a veto-proof majority in both houses, contrary to what Congress expected when it reformed the relevant statutory scheme in 1976.

Fundamentally, though, what we will have witnessed if the veto is upheld is the failure of enough congressional Republicans to stand up to Mr. Trump and the voter base that responds to his bursts of demagoguery on Twitter, even at the expense of their constitutional authority and erstwhile principles.

To be sure, 12 senators, or about 23 percent of the GOP caucus, voted against Mr. Trump — an improvement over the 6.6 percent of House Republicans who mustered that much gumption. These Senate defectors — Lamar Alexander (Tenn.), Roy Blunt (Mo.), Susan Collins (Maine), Mike Lee (Utah), Jerry Moran (Kan.), Lisa Murkowski (Alaska), Rand Paul (Ky.), Rob Portman (Ohio), Mitt Romney (Utah), Marco Rubio (Fla.), Patrick J. Toomey (Pa.) and Roger Wicker (Miss.) — deserve praise for teaming with Democrats to hand Mr. Trump a symbolic rebuke, the first of its kind during his presidency. But most of Mr. Trump’s party remained unwilling to do the right thing, despite their zeal in denouncing executive overreach when the alleged overreacher was President Barack Obama.

Especially disappointing were the votes in favor of Mr. Trump’s emergency by Republican Sens. Thom Tillis (N.C.), Ted Cruz (Tex.) and Ben Sasse (Neb.). All three style themselves guardians of conservative constitutionalism; Mr. Tillis had gone so far as to assert, in a Feb. 25 op-ed for The Post, that his conservative principles would not let him uphold Mr. Trump’s emergency declaration. With the party’s ultras talking about a primary challenge when he is up for reelection in 2020, however, Mr. Tillis flip-flopped, claiming that now there is a “serious discussion” of amending laws to prevent a repeat of this situation.

Opponents of the emergency declaration now put their hope in a long-shot court challenge. It remains possible, at least in theory, for Congress to do the job by overriding a veto on a bipartisan basis. Ironically, it was Mr. Tillis who best articulated the stakes. “There is no intellectual honesty in now turning around and arguing that there’s an imaginary asterisk attached to executive overreach,” Mr. Tillis wrote in The Post, “that it’s acceptable for my party but not thy party.”