If anything's constant in American political life, it's the stable two-party system, jostled occasionally by third-party presidential challengers such as Ross Perot in 1992 and 1996 or Theodore Roosevelt in 1912.
Yet, more rarely, at times of extreme political flux, this society has broken up into four parties.
In 1948, the first post-World War II presidential election year, Republicans ran against three Democratic party factions: Harry Truman's pro-New Deal, anti- Communist majority wing (which won in November), a Southern-based segregationist offshoot led by Strom Thurmond, and pro-Communist bolters headed by former vice president Henry Wallace. The latter two polled more than 1.1 million votes each out of 48 million cast; Thurmond got 39 electoral votes.
In 1860, as the country and its political parties came apart over slavery, the Republicans, northern and southern Democrats and a residual Whig body called the Constitutional Unionists fielded candidates for the White House. All four captured electoral votes; Abe Lincoln's victory gave way to the Civil War.
Might we be headed toward another four-party moment? There are two reasons to say "yes." The first is the argument presented in conservative scholar James Piereson's provocative new collection of essays, "Shattered Consensus." As the title suggests, Piereson believes, along with many other political analysts, that this country's current political malaise represents an unstable new normal - that the formerly consensual postwar political order has "at length produced a sorting-out of Americans into conflicting and sometimes hostile political, social and geographical groups. The most obvious historical precedent we have for such a configuration is the one that developed in the 1850s."
Causing this breakdown, in Piereson's view, are the mounting inefficiency and cost of government, which fuel conservative demands for radical shrinkage of the state - coupled with the decreasing productivity and perceived unfairness of the economic system, which fuel a liberal demand for radical expansion of the state.
Long gone is the optimistic Keynesian belief in managed capitalism, which, according to Piereson, supported the centrist two-party politics of the postwar era. Even recent signs of economic recovery, including a low 5.3 percent unemployment rate, can't seem to revive confidence in either government or business.
The second reason to speculate on a four-party moment is the course of the current presidential campaign, which has been more about the internal struggles of Republicans and Democrats than the differences between the two parties.
In each party, the source of division is an ideologically purist voter "base" (left-wing Democrats, right-wing Republicans) fed up with what it perceives to be the past corrupt compromises of the party "establishment," which allegedly takes its votes for granted.
To be sure, this dynamic is especially pronounced on the Republican side, where the tea party movement against Republicans In Name Only started at the end of the George W. Bush administration.
GOP unrest is so advanced that even a candidate who isn't actually that conservative - Donald Trump - has managed to exploit it, probably only temporarily, through sheer force of defiant attitude.
But the surprising success of Sen. Bernie Sanders's, I-Vt., left-wing campaign against presumptive nominee Hillary Clinton - he's leading in the latest New Hampshire primary poll - shows there is populist unhappiness in Democratic ranks as well.
The usual methods of co-optation by which Republican and Democratic politicians have maintained cohesion within their respective big tents are proving remarkably ineffective.
GOP candidates' concessions to the right only seem to feed the demand for more; ditto for the attempted leftward movements not only of Clinton but also of former Maryland Gov. Martin O'Malley.
In short, a lot of the energy in politics now comes from those who reject, well, politics, at least politics as we know it. They insist on simple solutions to complex problems - whether it's Sanders's call for free state college tuition, paid for by a tax on stock traders, or Trump's promise to build a wall along the Mexican border, paid for by Mexico.
The likely scenario for 2016 is that Republicans and Democrats will hang together, though the specific ideological agenda that emerges from their respective primaries is still very much up for grabs. The incentives to avoid actual party crack-up are overwhelming; and the resources at the disposal of establishment Republicans and Democrats remain formidable.
Not even Piereson extends his analogy between present-day party politics and those of the 1850s to include a threat of civil war.
Rather, he foresees, all too plausibly, "an extended period of stalemate as each party blocks the agenda of the other, and a majority fails to form over any single approach to national challenges." It would take a crisis to break the impasse, Piereson writes - a sobering thought, given how many crises we've already been through.
Lane is a member of The Washington Post's editorial board.