Sen. Lamar Alexander (R-Tenn.) has expressed concern about President Trump’s emergency declaration. (Al Drago/Bloomberg)

THE HOUSE HAS voted. Now the Senate must decide whether it will defend its prerogatives from this wayward White House. The primary question for senators is not whether they favor construction of a border wall. It is whether any president, having failed to persuade Congress to appropriate money for something he favors, should be able to ignore Congress and just spend public funds.

President Trump declared a national emergency to reprogram $3.6 billion in military construction funds toward building his border wall. Reality on the border — illegal crossings are way down; illegal drugs mostly come through crowded ports of entry, not across open stretches of desert; the number of illegal immigrants in the United States has been dropping — shows there is no sudden crisis that would justify an emergency declaration. As Mr. Trump himself said, “I didn’t need to do this, but I’d rather do it much faster.” The only crisis here is that the president, stymied by Congress, feels political pressure to deliver on his signature campaign promise.

The House on Tuesday voted 245 to 182 to countermand the president’s emergency declaration. A principled but disappointingly meager band of 13 Republicans broke ranks to join the Democratic majority in defending their institution’s core prerogative to decide how taxpayer money is spent. Now the Senate must vote on the measure, which cannot be filibustered, because the National Emergencies Act of 1976 requires such votes to be fast-tracked. Already, three Republican senators have promised to vote to void Mr. Trump’s emergency declaration. One more, along with every Democrat, would be enough to undo the pretend emergency, and many GOP senators appear troubled about Mr. Trump’s end run around Congress — from the mild-mannered Sen. Lamar Alexander (Tenn.) to conservative firebrand Sen. Ted Cruz (Tex.).

If the resolution is approved, the president could still veto it, and a two-thirds majority would be needed to override in both chambers. Even if a veto is upheld, however, there is value in passing the disapproval resolution.

If both houses of Congress were to rebuke Mr. Trump’s emergency declaration, it would send a strong signal to the courts that lawmakers do not consider the nonexistent border crisis to be an emergency — and that the president is breaking the trust Congress placed in the executive branch when lawmakers delegated emergency powers to the White House. Legal proceedings are already underway. Such a vote might give judges more leeway to rule narrowly against Mr. Trump without abridging the president’s power to act in a true emergency later.

The resolution’s passage might also deter future presidents from playing similar games with emergency powers. It is crucial that emergency declarations not become another partisan tool. When presidents need to mobilize people, resources and political will, an emergency declaration should inspire cooperation, not be easily dismissed.

For the good of the country — and out of institutional self-respect — senators should reject Mr. Trump’s emergency declaration.