Sucks to be you, Dick Lugar.

The defeat of Richard Lugar in the Indiana Republican Senate primary is the kind of event that would have been shocking just a half-dozen years ago, and has since grown routine. Incumbent senators used to have almost no fear that they might be deposed by members of their own party for ideological or partisan deviations, and now that threat has become the most powerful disciplinary tool available to activists. And it’s a tool, moreover, that is being deployed asymmetrically – the homogeneously conservative Republican Party has winnowed out virtually all its moderates, while the Democratic Party remains a looser coalition of moderates and liberals.

The toppling of Lugar is a seminal event because he is such a long-standing and esteemed member of the Senate, and his deviations were so tiny. The chorus of complaints about the challenge to Lugar has blended together process and substance, and the two need to be disentangled.

First, the process: Indiana Republicans have a right to nominate a more conservative senator. They have no obligation to settle for a figure who might go off the reservation and make a deal if they don’t want that. Richard Lugar, of course, is upset – he likes Richard Lugar. He thinks Indiana Republicans should like him. But party primaries are part of the democratic process, and senators’ desire to avoid subjecting themselves to this part of the democratic process shouldn’t be confused with a principle the rest of us ought to care about.

The substance, however, is a different matter. The main affliction of American politics is not a lack of bipartisanship. It is, as the former advocates of on-the-one-hand, on-the-other-ism Thomas Mann and Norman Ornstein argue in their new book, the collision of an increasingly radical Republican Party with a creaky political system poorly equipped to handle unified, fanatical parties.

Lugar, in a bitter concession statement, lashes opponent Richard Mourdock for his frank and open advocacy of totalistic partisanship. Yet in the classic manner of right-thinking advocates of bipartisanship, he makes sure to insist that both parties are to blame:

I don’t remember a time when so many topics have become politically unmentionable in one party or the other. Republicans cannot admit to any nuance in policy on climate change. Republican members are now expected to take pledges against any tax increases. For two consecutive Presidential nomination cycles, GOP candidates competed with one another to express the most strident anti-immigration view, even at the risk of alienating a huge voting bloc. Similarly, most Democrats are constrained when talking about such issues as entitlement cuts, tort reform, and trade agreements.

Lugar lists three issues here as evidence that the Democrats are equally in hoc to unflinching dogma as are Republicans: entitlement cuts, tort reform, and trade agreements. Certainly some Democrats take liberal positions on these issues. But, in fact, Barack Obama has enacted entitlement cuts and proposed others, has offered to support tort reform multiple times, and has signed several free trade agreements. If these issues are the equivalent to the Republican mania on climate change, immigration, and taxes, it shows just how asymmetrical the two parties are at the moment.

The most important and alarming facet of Lugar’s defeat, and a factor whose importance is being overlooked at the moment, is one of the things Mourdock cited against him: Lugar voted to confirm two of Obama’s Supreme Court nominees. Obviously, Lugar would not have chosen to nominate an Elena Kagan or a Sonia Sotomayor. But he was following a longstanding practice of extending presidents wide ideological latitude on their Supreme Court picks. In the absence of corruption, lack of qualifications, or unusual ideological extremism, Democratic presidents have always been allowed to pick liberal justices, and Republican presidents conservative ones. That’s not a law. It’s just a social norm.

But the social norms that previously kept the parties from exercising power have fallen one by one. Under Obama’s presidency, Republicans have gone to unprecedented lengths to block completely uncontroversial appointments, paralyzing the government and using the power to paralyze government to nullify duly passed laws. It is bringing on an approaching crisis of American government.

The social norm against blocking qualified, mainstream Supreme Court nominees is one of the few remaining weapons the Republican Party has left lying on the ground. But if Republican senators attribute Lugar’s defeat even in part to those votes for Kagan and Sotomayor, which seems to be the case, what incentive do they have to vote for another Obama nominee? And then what will happen if he gets another vacancy to fill – will Republican senators allow him to seat any recognizably Democratic jurist? Especially as the Supreme Court interjects itself more forcefully into partisan disputes like health care, will it become commonplace for the Court to have several vacancies owing to gridlock, for the whole legitimacy of the institution to collapse?

It’s possible that nothing important will come of the Lugar-Mourdock primary. But it is just as easy to see in it the frightening outlines of a future systemic crisis.