Even more fundamentally than this, a Trump presidency could initiate a dramatic shift in the institutional balance of power between the executive and Congress. The legislature’s chronic weakness in recent years has rested in part on the growth of partisan polarization, which incentivizes the president’s co-partisans in Congress not just to acquiesce to, but also encourage, acts of executive overreach. The Obama years offer rich testimony to this distortion of the Constitution. Trump would, by contrast, have the full buy-in of neither of the two main parties. Congress would then wield formidable power, presenting a vital check on any attempts by Trump to govern unencumbered. For all the recent discussion of the decline of Congress, the fact remains that the total formal power assigned to the legislature under the Constitution is insurmountable. With a veto-proof majority, it cannot be stopped. On some matters, at least, parts of the Republican party could ally with Democrats, and Paul Ryan could end up at the very center of policy-making on some issues.

Though Trump still has little backing from Republicans in Congress—just eight representatives and one sitting senator at the last count—he will of course attract further support if nominated and still more if elected. A significant part of the GOP might adopt a wait-and-see approach, assessing how Trump performs and lending support on a case-by-case basis. That would still represent a shift from anything we have seen in recent times. The power of Congress would suddenly look very different. It could even open the way to a more constitutional relationship between president and Congress, rather than one distorted by parties.

Trump, meanwhile, might fear finding himself playing the role of a latter-day Andrew Johnson, a president without a firm basis of support in a political party. As this analogy suggests, looming over a Trump presidency is the possibility of impeachment, Congress’s own nuclear option. It is the mere fact that such a weapon could be used, not that it would, that alters the dynamic so profoundly. Here we see why the contingency planning of the kind Kahn advocated must begin sooner rather than later. With a vice president ready and able to assume presidential duties, a safe transition would become altogether less unthinkable. A credible vice president could turn out to be the nation’s, not the president’s, trump card.