If Democrats tried to pack the Court, Republicans could respond by packing in turn when they next won a majority. But they could also respond by nullification — i.e. red states could decide a federal ruling is unconstitutional and simply refuse to comply — as happened in the 1830s. That would impend a far deeper crisis of legitimacy, the resolution of which would be imperative for both parties — and for that reason would likely mean a compromise far friendlier to Republicans than what many liberal Democrats want from the Court. If the past is prologue, re-establishing federal authority would almost certainly require appeasement of the aggrieved states, which would mean scaling back the Court’s scope of intervention in their affairs. And while the delegitimization of any branch of government is something that should give temperamental conservatives pause, the fact that the result would be far more obviously and directly detrimental to liberal hopes gives them ample reason to call Democrats’ bluff in this case. That’s how as temperamentally conservative a commentator as The New York Times’ Ross Douthat can wind up saying to liberals, in so many words: Go for yours.

If Democrats want to fight back, then, they need to present a more credible set of carrots and sticks. There are Republican senators who still blanche at the prospect of a full-blown crisis of legitimacy. Chuck Grassley of Iowa is one; Mitt Romney of Utah is another. What can the Democrats offer them that they could sell to their own voters as a better, stabler deal than momentary advantage, even in the pursuit of a generations-long GOP goal?