Our political system has been upended, but has it been fundamentally altered?

Now, finally, there is a logic — a strategic and even ideological rationale — for a presidential campaign that has shattered all the assumptions of logic, all the strategic and ideological precedents, of our politics.

With the final presidential debate now in the swiftly receding past, the final balloting now swiftly approaching, the banalities and bathos now reaching their inevitable but welcome ends, we see clearly what this election is about.

The ultimate disrupter now has a limited amount of time to disrupt the process that has given him the Republican presidential nomination but now threatens to deny him the ultimate prize. The consummate curator of the conventional now faces the limited challenge of running out the clock on an election that once seemed hers to lose, then seemed possible to see her losing, and now seems within her grasp.

In sports terms, we have the two-minute drill against a ‘‘prevent’’ defense. In music terms, we have a master of the scat line against a virtuoso of the sonata-allegro. Manhattan businessman Donald J. Trump and former Secretary of State Hillary Rodham Clinton are struggling, respectively but not respectfully, to produce and to prevent a surprise symphony. (Joseph Haydn on his 1791 work of that name: ‘‘I was interested in surprising the public with something new, and in making a brilliant debut.’’)

The remarkable thing about this dispiriting campaign is that neither of the roles these two pugilists have assumed is natural — they are acquired, though to say that they are an acquired taste is to give to them more honor than they deserve. Indeed, these roles go against the instincts and histories of both of the principals, to say nothing of their fast-vanishing principles.

Consider Mr. Trump’s background: a businessman, albeit with a showman’s flair. Whether on Wall Street or Main Street, or on the downtown avenues and coastal resorts from Bay Street in Toronto to Ocean Boulevard in Palm Beach, business executives ordinarily prize stability — and yet Mr. Trump is the personification of disruption.

Consider Ms. Clinton’s background, from the New England afternoons of her anti-war activism to the commencement morning of her Wellesley education to her stereotype-smashing years as first lady in the governor’s mansion in Little Rock and in the White House. Always, until now, she has been the sworn enemy of the status quo — and yet in this race she is the personification of the ancien regime.

There never has been a series of presidential primaries, there has never been a set of presidential debates, there has never been a general-election campaign, remotely like this.

That may seem a facile statement in a relatively young country with a Heinz-variety 57 presidential elections. But the United States is a mature democracy — the phrase need not prompt a chorus of snickers — and its recent dozen or so contests have settled into a reliable pattern.

In tone and timbre, in insults and importations, Mr. Trump and Ms. Clinton have shattered that pattern. No one — not even the combatants themselves — can admire the depths to which this campaign has descended, tarnishing our political system. Whether this proves to be the road not taken in the future will make all the difference in the future of our politics.

But now, the more immediate path ahead seems clear. The disrupter hopes to create one more disruption — in the narrative, and then in the momentum, of this race. The conventional candidate must preserve its motion and rhythm. Here the businessman is the provocateur. Here the longtime agitator has the stake in the status quo.

All this was on display in Wednesday night’s debate and in the combat that followed. Mr. Trump taunted Ms. Clinton by declaring, ‘‘Such a nasty woman.’’ Ms. Clinton characterized his refusal to say he would accept the verdict of the election as ‘‘horrifying.’’ He accused her of “criminal intrigue.” She suggested he was a ‘‘puppet” of Vladimir Putin.

Earlier debates are remembered for one line at most — “There you go again, Mr. President” (former Gov. Ronald Reagan to President Jimmy Carter, 1976) or ‘‘I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent’s youth and inexperience” (Mr. Reagan to former Vice President Walter F. Mondale, 1984). There have been so many thrusts and parries in the 2016 events that the conventions of presidential debates themselves lie, like so many other elements of our politics, in ruins.

This has been a campaign not of great mobilizations across established fronts but instead of improvised explosive devices — and we may look back on this moment in our national passage either as a diversion or (and here is a phrase employed only in regret) the new normal.

In transition are the party alignments (blue-collar voters to the Republican nominee, a patina of elitism to the Democratic coalition); the traditional forms of presidential politics (party loyalists not always rallying behind their nominee); the language of politics (Ms. Clinton said her rival ‘‘choked’’ in his late-summer visit to Mexico; Mr. Trump said his opponent ‘‘shouldn’t be allowed to run’’ because she was ‘‘guilty of a very, very serious crime’’); the content of politics (whether one candidate was a physical abuser of women and whether the other was an intimidator of women); and the conduct of debates (interruptions, menacing background movements).

Some of these elements are of substantial political moment: If, for example, the Democrats are losing their New Deal coalition, and if the Republicans are adopting a fresh appeal to working Americans, we may be witnessing a landmark turning point.

These sorts of disruptions upend political assumptions, and they have occurred, among other times, in 1828 with Andrew Jackson, in 1980 with Ronald Reagan and during the Al Smith/​Franklin Roosevelt transition of 1928 through 1936. They set in motion important intellectual and ideological forces.

We may be witnessing such a moment this fall. But we also may be witnessing merely the coarsening of our culture — and the further diminution of our politics. The newest Rasmussen Reports survey indicates that half of likely voters say they will be selecting the lesser of two evils. It is an election of the lesser angels of our nature.

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.