I argued recently that political fundamentals point to a strong Biden reelection in 2024: The economy is growing, employment is rising, and Republican culture-warring is alienating crucial groups of voters. But big trends can be punctuated by unexpected events—the X factors that bump history off its predicted course.

The 2016 election cycle was dominated by two important last-minute shocks: Donald Trump’s Access Hollywood recording and FBI Director James Comey’s announcement that he was reopening an investigation into Hillary Clinton’s email practices. One proved damaging; one did not.

X factors don’t appear out of nowhere. WikiLeaks had dumped one load of Russian-hacked materials in summer 2016, as the presidential race warmed up; no surprise the group released another load in the fall, priming Comey’s announcement. For an audio clip to emerge offering evidence of Trump’s sexual misconduct was no great surprise either, even though the crude boasting in his own voice temporarily jolted senior Republican leaders such as Paul Ryan and Mike Pence.

[David Frum: The coming Biden blowout]

For 2024, too, we can discern the outline of possible X factors. Still, the idea of a thing is never the same as the thing itself, which cannot be fully understood until it materializes.

One potential factor is Joe Biden’s health. Only about a third of Americans feel confident that Biden is up to the physical and mental demands of the presidency, according to the most recent Washington Post/ABC poll.

This pervasive unease has already created a potential opportunity for Biden’s Republican opponent, whomever that may be. Instead of targeting the safe and familiar Biden, that opponent can direct fire at Biden’s running mate: less known, easier to define. If the running mate is Kamala Harris, the sitting vice president, then Biden’s opponent will almost certainly try to exploit popular anxieties over race, sex, and immigration (both of Harris’s parents were foreign-born). Has some panel of California Democrats proposed multimillion-dollar reparations payouts to Black Americans? ​Blame Harris! Disorder on the New York subway system? Blame Harris! A trans influencer on a big-brand beer can? Blame Harris! A surge of asylum seekers at the U.S. border? Harris, Harris, Harris!

Presidential-reelection campaigns are organized to promote and defend the record of the president, not the vice president. That can create a vulnerability. The 2008 John McCain operation collapsed amid internal bickering when Democrats identified the Arizona senator’s running mate, Sarah Palin, as a liability.

That running-mate weakness will come under even greater pressure if Biden suffers any negative health event between now and Election Day. Senate Minority Leader Mitch McConnell, age 81, was recently incapacitated for several weeks by an injury from a fall. The Senate Judiciary Committee is paralyzed because of the infirmity of Senator Dianne Feinstein, age 89. Democrats lost the chance to replace Justice Ruth Bader Ginsburg with another liberal because Ginsburg refused to retire. If Biden has to stop campaigning because of even a twisted ankle or a respiratory infection, never mind anything more serious, all of the doubts about his fitness—and Harris’s—will surge to the fore.

[Yair Rosenberg: The ice-cream theory of Joe Biden’s success]

Biden himself is not handling the age issue patiently or with good humor. Pressed by MSNBC’s Stephanie Ruhle last week, he responded with tight-lipped irritation: “I have acquired a hell of a lot of wisdom. I know more than the vast majority of people. I’m more experienced than anybody who has ever run for the office and I think I’ve proven myself to be honorable as well as also effective.”

If Democrats have their own concerns about Biden’s possible inability to serve a full second term, and the likelihood of a President Harris by default before 2028, they show no sign of doing anything about it. When Franklin D. Roosevelt sought a fourth term in 1944, leaders of his party first forced him to dump his serving vice president, the erratic Henry Wallace, and then vetoed Roosevelt’s preferred alternative, James Byrnes of South Carolina. Byrnes was a segregationist who had left the Roman Catholic Church, potentially alienating northern liberals and Catholics. Party leaders wanted Harry Truman instead—and imposed their wish on Roosevelt. Their determination proved well founded. Nine months later, Roosevelt was dead.

Truman went on to win reelection, in his own right and against expectation. But the Democratic party of today has no similar mechanism to replace a poorly polling running mate with a stronger one without triggering a protracted spasm of accusation and counter-accusation—of racism, sexism, and the rest of the intra-progressive lexicon of grievance.  

X factors apply not just to Biden. The Republican campaign faces problems of its own: Trump is not much younger than Biden. But the risks that most thickly crowd around the GOP’s leading candidate are legal, not medical. Trump has already been indicted by the Manhattan district attorney. What if he’s convicted in that case, or indicted in additional possible cases being pursued by the Department of Justice and a Georgia district attorney?

Trump’s indictments have, thus far, generated a rally effect among his co-partisans, widening his lead over Florida Governor Ron DeSantis to 30 points in the month after. Trump’s famous confidence that his supporters would follow him even if he shot someone in the middle of Fifth Avenue seems vindicated.

[David Frum: Justice is coming for Donald Trump]

But the emphasis here is on thus far. More indictments may be coming. Trump is also engaged in a civil suit in which the underlying issue is an accusation that he raped one woman, backed by testimony that he sexually assaulted many more. As president, Trump could rely on some political cover because the sheer number of allegations of wrongdoing got jumbled together, confused people, and often canceled one another out. Whether accumulating indictments will now cancel out in the same way is not so clear—even less so if they turn into accumulating convictions, followed by sentences. It’s not inconceivable that Trump could be wearing an ankle bracelet when and if he delivers his acceptance address at the Republican National Convention.

If Trump receives a criminal conviction for sedition, conspiracy, or some other crime against American democracy, his most hard-core supporters might turn to extralegal or even violent forms of action, as happened on January 6, 2021. Such a repudiation of the rule of law could create an internal security challenge for the United States. At least some of the spate of mass shootings since 2021 can plausibly be interpreted as a subideological insurgency against legal authority. That’s another X factor to worry about, one protected by the way many conservatives have inscribed gun rights at the very center of their cultural identity.  

The immediate X factor is whether a convicted Trump can remain viable in presidential politics. The answer has to be no. Trump heads a coalition that includes a lot of people who do not like him very much. Multiple polls find that one-fifth to one-third of self-identified Republicans hold unfavorable opinions of Trump, depending on when and how the question is posed. In November 2021, Marquette found that 40 percent of Republicans wish that Trump would not run again. Quinnipiac reported in November 2022 that a quarter of Republicans regard Trump’s influence as negative for their party. In April 2023, NBC showed that a quarter of Republicans want a nominee who is not distracted by his personal legal troubles. In a May Washington Post/ABC poll, 22 percent of Republicans and Republican leaners said they would be “dissatisfied” if Trump were nominated in 2024.

[David Axelrod: Why neither party can escape Trump]

Trump won 45.9 percent of the vote in 2016 and 46.8 percent in 2020—about the same popular-vote share as Michael Dukakis won in 1988, and less than Al Gore’s in 2000, John Kerry’s in 2004, and Mitt Romney’s in 2012, all of whom were, of course, the losing candidate in their respective race. Trump does not start his third presidential contest with a large margin to spare. The American electoral system is tilted in favor of rural and conservative candidates—but not enough to save a presidential candidate who falls below Trump’s 2016 and 2020 levels of support.

X factors can be events entirely unrelated to the candidates. Perhaps congressional Republicans will mishandle their debt-ceiling gambit and plunge the U.S. economy into crisis and depression. Perhaps, if facing defeat in Ukraine, the Russians will act on their threat to use nuclear weapons. Perhaps the scheme of the former Trump strategist Steve Bannon to mount a spoiler campaign with Robert F. Kennedy Jr. in 2024 will draw away more Democratic votes than Kanye West’s equivalent stunt did in 2020.

The X factors have to be weighed. But they have to be weighed against all of the other factors that point, at present, toward the conventional wisdom of Biden’s reelection.