LACONIA, N.H. - U.S. presidential elections have been rocked in recent years by economic disaster, stunning gaffes, secret video and a pandemic. But for all the tumult that defined those campaigns, the volatility surrounding this year’s presidential contest has few modern parallels, posing profound challenges to the future of American democracy.

Not since the Supreme Court effectively decided the 2000 campaign in favor of Republican George W. Bush has the judiciary been so intertwined with presidential politics.

In the coming weeks, the high court is expected to weigh whether states can ban former President Donald Trump from the ballot for his role in leading the Jan. 6, 2021, insurrection at the U.S. Capitol. Meanwhile, a federal appeals court is weighing Mr. Trump’s argument that he’s immune from prosecution.

The maneuvers are unfolding as prosecutors from New York to Washington and Atlanta move forward with 91 indictments across four criminal cases involving everything from Mr. Trump’s part in the insurrection to his efforts to overturn the 2020 election and his hush money paid to a porn actress.

Depending on how Mr. Trump’s appeals play out, he could be due in court as early as March 4, the day before Super Tuesday, raising the unprecedented prospect that he could close in on the GOP nomination from a courtroom.

On the Democratic side, President Biden is seeking re-election as the high inflation that defined much of his first term appears to be easing.

But that has done little to assuage restless voters or ease widespread concerns in both parties that, at 81, he’s simply too old for the job.

And at least three serious candidates who have launched outsider presidential bids threaten to scramble the campaign and eat into the support from independent voters who were critical to President Biden’s success in 2020.

Facing such uncertainty, few expect the traditional rules of politics to apply in 2024. Jim Messina, who managed former President Obama’s re-election, said Mr. Trump could very well defeat President Biden in the fall, even if the former president is in prison.

“We just don’t know,” Mr. Messina said. “Everyone in the world knows, especially me, that this election is going to be really, really close.”

The results will have long-term implications on everything from the future of abortion rights and immigration policy to the role of the U.S. in the world. A Trump victory would raise the possibility of the U.S. largely abandoning Ukraine as it seeks to repel Russia’s invasion. Domestic politics could also test President Biden’s commitment to Israel, a policy that threatens to erode his standing with young voters and people of color who are critical elements of his coalition.

One of the few certainties at this point is that President Biden is a virtual lock to be the Democratic nominee again, facing only token opposition

in this year’s primary despite overwhelming concerns within his own party about his physical and mental fitness. And though a few rivals are fighting furiously to stop Mr. Trump, he is well positioned to win the GOP nomination for the third consecutive election.

The strength of the GOP opposition to Mr. Trump will become more clear on Jan. 15 when the Iowa caucuses launch the nomination process. Mr. Trump holds a commanding lead in most national polls, although former U.N. Ambassador Nikki Haley and Florida Gov. Ron DeSantis are fighting to stop him.

That hasn’t been easy, however, as Gov. DeSantis has struggled to connect with voters and has embraced culture war topics that often left him competing for the same base of support as Mr. Trump. And Ms. Haley’s pitch as a more sensible, moderate candidate was threatened last week when she was pressed on the cause of the Civil War and didn’t mention slavery.

Allies of Gov. DeSantis and Ms. Haley privately concede that their best chance to wrestle the nomination away from Mr. Trump would come in a long-shot push for a contested convention in Wisconsin in July.

Many leaders in both parties are already convinced that Mr. Trump will be the GOP nominee. More than 90 House Republicans, 18 senators and seven governors have endorsed Mr. Trump. Ms. Haley and Gov. DeSantis have secured the endorsements of just six House Republicans, no senators and two governors combined.

“This will be one of the earliest primaries wrapped up in my lifetime,” Rep. Elise Stefanik, R-N.Y., who endorsed Trump back in November 2022, said in an interview. “I’m already focused on the general election. ... There is going to be a political earthquake next November.”

Public polling strongly suggests that voters do not want a rematch between Mr. Trump and President Biden.

Most U.S. adults overall (56%) would be “very” or “somewhat” dissatisfied with President Biden as the Democratic presidential nominee

in 2024, according to a poll conducted last month by The Associated Press-NORC Center for Public Affairs Research.

A similar majority (58%) said they would be very or somewhat dissatisfied with Mr. Trump as the GOP’s pick.

Perhaps because of such apathy, some voters simply don’t believe President Biden and Mr. Trump will be on the general election ballot, despite strong evidence to the contrary.

That’s an idea that conservative strategist Sarah Longwell, who founded the Republican Accountability Project, says she hears regularly during weekly focus groups with voters across the political spectrum.

“Voters really aren’t thinking about it, so they don’t see the thing that’s coming right at us — the most likely scenario, which is Trump vs. Biden,” Ms. Longwell said. “But Trump is so dangerous. ... I wish the level of urgency from everybody matched the reality of where we are headed.”

While concerns about President Biden are centered on his age, Mr. Trump has increasingly embraced authoritarian messages that serve as clear warnings of his plans to dismantle democratic norms if he returns to the White House.

Echoing strongmen leaders throughout history, Mr. Trump has framed his campaign as one of retribution and has spoken openly about using the power of government to pursue his political enemies.

He has repeatedly harnessed rhetoric once used by Adolf Hitler to argue that immigrants entering the U.S. illegally are “poisoning the blood of our country.” He said on Fox News last month that he would not be a dictator “except for day one.”

And he shared a word cloud last week to his social media account highlighting words