Now that President Trump has declared a national emergency to circumvent Congress and build his wall, Republican legislators face a time for choosing: Support Trump or the rule of law.

The upcoming vote on the emergency order will be a defining—if not the defining—vote of this Congress and it will test the GOP’s commitment both to constitutional norms and to limited government.

With Majority Leader Mitch McConnell’s abdication of his institutional and constitutional responsibility, who will step forward to make the case and organize other Republican senators to uphold the rule of law? Who will speak for the non-autocratic wing of the Republican party?

In the short term, Trump’s move may give the illusory impression of a win: It keeps the government open and Trump and his supporters can pretend that he is fulfilling a campaign promise. (Except for the awkward fact that Mexico is not paying for the wall, the wall wouldn’t go from sea to shining sea, and the proposed wall would not really be a wall.)

But it is a fatal mistake to view this moment in context of the news cycle. Republicans in Congress need to understand the historical and institutional implications.

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Conservatives once understood the concept of “unintended consequences,” and they are not only going to get a refresher course, they are likely to get it good and hard. How bad could it be? The worst-case scenario for the GOP is if Trump’s power grab succeeds. If the courts uphold the president’s sweeping power to declare emergencies and abrogate Congress’s Article I powers, the damage to the constitutional equilibrium is likely to last for generations.

It hardly takes a wizard to imagine how those powers might be exercised by a Democratic president—on climate change, gun rights, healthcare, even trade.

Writing in National Review, David French lays out the stakes:

[This] is an attempted abuse of the constitutional order that is justified mainly by the existence of previous successful abuses of the constitutional order. Each abuse builds on the next; hypocrisy builds on hypocrisy. The only clear winner is the imperial presidency. The loser is our constitutional republic. And each Trump fan cheering his raw power grab will be a furious partisan when the next Democratic president builds on Trump’s abuse.

We agree. And Republican senators warned Trump against the move he took today precisely because they also understand the dangerous precedent it would set. But warnings and furrowed brows no longer suffice. Republican members of Congress need to remember what the Constitution demands of them.

Lawsuits against the emergency declaration may well be appropriate and necessary and extended litigation seems inevitable. But leaving the matter to the courts is suboptimal first, because it places the power in the hands of unelected judges and second because even the act of relying on the courts will, all on its own, further diminish the power of the legislature.

The first priority for defenders of the Constitution and the rule of law should be pressuring members of Congress to act on behalf of their own powers.

It is critically important to have a vote in both Houses on the emergency declaration. Of course, Trump will veto the congressional action, but even if the votes to override aren’t there, it is right and proper to put our elected officials on record at this moment of truth. They will have to choose whether they are willing to cross this red line for a fictional emergency and an imaginary wall.

We hope that this will be the moment when senators such as Mitt Romney, Susan Collins, Marco Rubio, and Ron Johnson put their constitutional obligations ahead of their party loyalties. But all members of Congress take the same oath of office (see Title 5, Section 3331 of the United States Code):

“I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion, and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God.”

This is their moment to live up to that oath.