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.)