Legitimacy vs. Politics
The Founders, bigoted and oligarchical (and--don't tell the Tea Party--elitist) though they might have been, were pretty smart guys. George Washington saw at the outset that factional politics could undermine the efficacy of representative government, and we can't say he didn't warn us:

All obstructions to the execution of the Laws, all combinations and associations, under whatever plausible character, with the real design to direct, control, counteract, or awe the regular deliberation and action of the constituted authorities, are destructive of this fundamental principle, and of fatal tendency.—They serve to organize faction, to give it an artificial and extraordinary force—to put in the place of the delegated will of the nation, the will of a party;—often a small but artful and enterprising minority of the community;—and, according to the alternate triumphs of different parties, to make the public administration the mirror of the ill-concerted and incongruous projects of faction, rather than the organ of consistent and wholesome plans digested by common councils, and modified by mutual interests.—However combinations or associations of the above descriptions may now and then answer popular ends, they are likely, in the course of time and things, to become potent engines, by which cunning, ambitious, and unprincipled men will be enabled to subvert the Power of the People, and to usurp for themselves the reins of Government; destroying afterwards the very engines, which have lifted them to unjust dominion.—
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The alternate domination of one faction over another, sharpened by the spirit of revenge, natural to party dissension, which in different ages and countries has perpetrated the most horrid enormities, is itself a frightful despotism.—But this leads at length to a more formal and permanent despotism.—The disorders and miseries, which result, gradually incline the minds of men to seek security and repose in the absolute power of an Individual; and sooner or later the chief of some prevailing faction, more able or more fortunate than his competitors, turns this disposition to the purposes of his own elevation, on the ruins of Public Liberty.

Without looking forward to an extremity of this kind, (which nevertheless ought not to be entirely out of sight), the common and continual mischiefs of the spirit of Party are sufficient to make it the interest and duty of a wise people to discourage and restrain it.

It serves always to distract the Public Councils, and enfeeble the Public administration. It agitates the community with ill-founded jealousies and false alarms, kindles the animosity of one part against another, foments occasionally riot and insurrection.

Facing the prospect of an all but explicitly partisan Supreme Court decision that seems very likely to overturn part or all of the Affordable Care Act, and poking through the wreckage of last summer's near-miss to strike a "grand bargain" on the national debt, it's difficult to reach a conclusion other than that partisan advantage has entirely overshadowed the larger national interest. Or maybe it's that in the eyes of the partisans, there's no daylight at all between what's good for Our Team and what's good for the country. Certainly there's not much consideration even for the idea that the Other Side might have a valid point.

It's not difficult to understand how we got here. The disappearance of conservative Democrats and liberal Republicans is now complete, and there aren't even many moderate Republicans remaining. Epistemic closure is pretty much a done deal on the right, and those on the left--I'll admit myself at least somewhat guilty here, as the links on this page suggest--also tend to choose their own validating sources.

But while we now have a parliamentary orientation with monolithic ideologies in Manichean struggle, our governance institutions remain premised on accommodation. Thanks to all the choke points built into the system, the parties can't really implement their programs even when they have full control, as the Republicans did from 2003-2006 and the Democrats did in 2009-10. Add in how closely balanced the sides, and the dream of a "final victory" in which, say, the Democrats impose single payer healthcare and raise taxes back to pre-Reagan levels, or the Republicans eliminate five federal agencies, privatize Social Security and turn Medicare into a voucher program, seems pretty implausible.

Ultimately, public faith in the system is predicated upon the demonstrated capacity of the system to keep faith with the public and deliver the goods of governance in a relatively rational and equitable manner. We know that the social contract started to fray in the '60s and '70s and has only continued to erode since then. The other piece, ability to serve public interests, is under ever greater pressure.

Speculation regarding the aftermath of the ACA being overturned mostly has focused on short-term political ramifications; whether the decision will illuminate President Obama as ineffectual or motivate his supporters to swarm forth in righteous fury. That's kind of the wrong question. The bigger question might be what happens when a step forward, however compromised or limited or preliminary, is erased for no good reason and without anything to replace it--when the suffering of millions is intensified for the gratification of "cunning, ambitious, and unprincipled men."