Americans are about to face a choice between two incumbent presidents. The idea sounds oxymoronic: a political version of the Pauli Exclusion Principle in physics, whereby two particles cannot occupy the same space at once. Yet that is precisely the situation — barring an act of God or the Obamas — in which we will soon find ourselves.

There hasn’t been a two-incumbent election between the major parties since 1892. That year, Republican president Benjamin Harrison faced the man whom he had defeated four years earlier: Democrat Grover Cleveland.

The 1888 election that had brought Harrison to power was unusual. There had been a split between the Electoral College and the popular vote, and despite his loss, Cleveland earned more votes than he’d won in victory in 1884.

The rematch in 1892 was a different story. By then, voters had tired of the inflationary effects of GOP protectionism. They returned Cleveland to office for a nonconsecutive second term.

In 2024, Donald Trump wants to play Cleveland to President Joe Biden’s Harrison. Trump, like Cleveland, won more votes losing in 2020 than he did winning in 2016. He, like Cleveland, leads a party whose geographic base is the South. And he, like Cleveland, has five children. The similarities — at least as I can count them — end there.

The precedent of 1892 is so distant that it hardly seems relevant. Our two-incumbent election is a genuine novelty. It pits a twice-impeached, criminally charged Republican against a deeply unpopular Democrat who faces his own impeachment inquiry and whose adult son is under federal indictment. All set against the backdrop of collapsing public trust, deteriorating world order, resurgent anti-Semitism, the interpenetration of the judicial system with domestic elections, myriad connections between former and current national security personnel and the major media “echo chamber,” America’s aggressive and cunning strategic adversaries, the legitimation of political violence, and a likelihood of constitutional crisis and domestic unrest. Harrison-Cleveland was placid by comparison. Even boring.

The two incumbents in 2024 have dominated the invisible primary. Trump has run not as if he were another run-of-the-mill contestant, but as if he currently held office and could claim the Republican nomination by right. None of his rivals have come close to his leads in either state or national polls. His risky decision not to appear on the debate stage looks, in retrospect, like a political masterstroke. Above all, Trump’s legal troubles caused Republicans to rally to his side. The charges confirmed, in the eyes of his supporters, that the system is rigged against them. The GOP primary could be over in three weeks.

Nor does Biden face a serious primary threat. Robert F. Kennedy Jr. decided to run as an independent, leaving Biden with massive leads over author Marianne Williamson, who garners 8 percent in national polls, and congressman Dean Phillips (Minn.), who is at 3 percent. Phillips, who is earnest and likable, says he is running because Joe Biden cannot win a general election.

And Phillips may be right. If this were a one-incumbent race that pitted Biden against a fresh Republican, Biden would be on his way to a landslide defeat. He begins 2024 with the lowest approval ratings of any modern president. Voters say that he is too old for the job, that things are “out of control,” and that he has made their lives worse. The Biden campaign has spent tens of millions of dollars in television advertising across swing states to counter these negative attitudes. The ads have had no effect. On the contrary: Biden’s position has worsened. Core Democratic constituencies — Hispanic voters, black voters, and 18- to 35-year-old voters — have turned against him.

Yet Biden has a chance. The Democratic coalition may be fracturing, but its pieces are not joining the GOP. Instead, disaffected Democrats are saying that they will stay home or that they will support RFK Jr. or Cornel West — if either man makes it onto state ballots.

Normally, a splintered electorate and a collapse in enthusiasm for the incumbent benefits the challenger. Not when the challenger is another incumbent. Not when that other incumbent is Donald Trump. The former president may be ahead, but his lead is narrow and within the margin of error.

Pollster Bill McInturff found that, unlike recent presidential contests, 2024 will be more about the challenger than the incumbent. In 2004, 61 percent of voters said their votes were more about George W. Bush than John Kerry. In 2012, 66 percent said their votes were more about Barack Obama than Mitt Romney. The 2020 election was more about Trump than about Biden, who was in his basement. And yet 57 percent of voters say their vote in 2024 will be more about Trump than about President Biden.

That is why Biden plans to campaign at Valley Forge this Friday, where he will deliver a speech attacking Trump as a threat to democracy. That is why Biden and Democrats plan to campaign just as they have in every election since 2016: portraying Trump and the MAGA movement as extremists bent on depriving the electorate of benefits, from guaranteed health insurance to abortion rights. It’s worked before — in 2018, 2020, and 2022.

Will it work again? Don’t look at me. I’m buying gold and MREs. Because in a two-incumbent election, nothing is guaranteed.

Matthew Continetti is a senior fellow at the American Enterprise Institute and the founding editor of The Washington Free Beacon. For more from the Free Beacon, sign up free of charge for the Morning Beacon email.