Once again, congressional Republicans are facing a gut-check moment, forced to choose between supporting and defending the Constitution or Donald J. Trump. It’s not looking good.

House Democrats introduced a joint resolution of disapproval on Friday aimed at canceling President Trump’s bogus national emergency at the southern border. Representative Joaquin Castro of Texas is leading the charge, and Speaker Nancy Pelosi is expediting the measure, which is on track for a floor vote on Tuesday . It is widely expected to pass the chamber, where Democrats have the majority.

Once the resolution clears the House, the Senate is required to hold its own vote within 18 days — meaning the majority leader, Mitch McConnell, cannot do his usual stonewalling. Even so, with Republicans in control there, and loath to irk the president, the resolution is widely expected to die. At that point, it will be up to the courts, where multiple suits have already been filed, to grapple with this out-of-control executive.

It is not too late to stop this legislative cop-out. Critical principles are at stake — Congress’s power of the purse, the separation of powers — that transcend any one declaration or leader. Members of both parties need to make clear that a presidential pique is not the same thing as a national emergency, that a president who fails to persuade Congress to support his priorities can’t then simply pursue them by fiat. Lawmakers who cannot rally themselves to this cause should stop pretending that they’re anything more than partisan automatons; they will have declared themselves members of a second-class branch of government.