100 days until the Iowa caucuses. What could possibly go wrong?

One hundred days before the Iowa caucuses, many in the punditry assume that the opening presidential contest will be the first step in a preordained march toward a Joe Biden v. Donald Trump rematch in November.

What could possibly go wrong?

Actually, a lot could change. Three months is plenty of time for an expanding United Auto Workers strike to take a toll on the economy, for Ukraine's fortunes to shift on the battlefield, for courtroom revelations and health scares, for front-runners to hit turbulence and challengers to gain momentum.

Enough time to upend what now seems all but certain.

Though the candidates who are ahead at this point often do end up being nominated for president, that doesn't always happen. At this point in the 2008 race, a CNN poll showed former New York Mayor Rudy Giuliani leading the Republican field as former senator Fred Thompson was fading. On the Democratic side, Sen. Hillary Clinton had built a 30-point lead that seemed almost indestructible.

But on Election Day, it was her then-distant rival, Illinois Sen. Barack Obama, who faced off against Arizona Sen. John McCain for the White House.

Here are five things that could shake up the presidential landscape before voters in Des Moines and Dubuque are raising their hands for one contender or another 100 days from this Saturday.

Revelations in the courtroom

A defiant Trump arrived in a New York courtroom Monday for his civil trial on charges of real estate fraud. Later this month, a trial in Georgia is set to begin for two of his co-defendants on criminal allegations of trying to overturn the 2020 election.

No former president has ever faced the legal perils Trump does now, with 91 criminal charges in four jurisdictions. So far, they seem to have strengthened his political position as many Republicans endorse his unfounded declarations that he is the victim of political vendettas orchestrated by the Biden administration's Justice Department.

As testimony begins under oath − and aired live from the Atlanta courtroom − new disclosures at least raise the possibility of challenging that narrative.

Abortion could bite in Iowa

Trump leads in Iowa, to be sure, by a substantial 23 percentage points in a Des Moines Register/NBC poll last month.

But evangelical Christians are a significant electoral force in the Hawkeye state, and opposition to abortion has long animated Republican politics there. Trump's declaration that Florida Gov. Ron DeSantis had made a "terrible mistake" by signing a ban on abortion at six weeks, his offer to negotiate a compromise with those who support access to abortion, and his refusal to endorse a federal abortion ban could cut against him here.

In the Iowa poll, DeSantis was second at 19%. South Carolina Sen. Tim Scott, who has run TV ads touting his support for restricting abortion, was third at 9%. Former Vice President Mike Pence, who has drawn the field's hardest line against access to abortion services, was at 6%.

Trump led at 42%. A 52% majority of those surveyed, however, said their minds were not made up and they could be persuaded to support another candidate.

A slip, a fall, a bad cold

A health crisis could complicate the campaigns of either the 80-year-old Biden or the 77-year-old Trump, both the oldest of the major contenders in their parties. A victory in 2024 would make either one of them the oldest nominee ever elected president, a distinction Biden already holds.

Concern about his age has emerged as particularly troublesome for Biden, and his fall after tripping over a sandbag on stage at the U.S. Air Force Academy in June created headlines, although he popped back up and seemed to sustain no injuries.

Since then, the White House has taken precautions to guard against another incident, including by using shorter stairs for Biden to enter and leave Air Force One.

Evidence in the impeachment hearings

California Rep. Kevin McCarthy, then the House speaker, launched an impeachment inquiry against Biden. The opening hearing last week got off to a stumbling start when two of the GOP's own witnesses said there wasn't enough evidence to warrant impeachment.

But the Biden family is making legal history, too.

Hunter Biden pleaded not guilty Tuesday in federal court in Delaware to violating federal gun laws. He is the first child of a sitting president to face criminal charges, and he may face additional allegations of tax evasion and other crimes.

But no one has produced evidence that the president misused his public authority or benefited financially from Hunter Biden's lobbying and business dealings. That is evidence the GOP House inquiry hopes to uncover.

A tightening GOP field, or a widening one

One of Trump's advantages in the Republican race is the multiplicity of his challengers. Eight of them passed the bar to participate in the first Republican debate, in Milwaukee, and seven of them were still doing well enough in fundraising and poll standings to make the second debate, in California.

A crowded field was a factor in his success in the competitive 2016 primaries, when a half-dozen contenders divided the anti-Trump vote.

This time, there are those calling on the longer shots to get out of the way, allowing voters who don't support Trump to consolidate behind a single alternative. Those appeals are likely to be directed at any candidates who don't qualify for the third primary debate next month, in Miami.

On the other hand, there is also an effort by some major Republican donors to make the field bigger by encouraging Virginia Gov. Glenn Youngkin to jump in. He hasn't ruled out the idea, although he says he won't decide until after the state legislative elections in November, in which control of the House of Delegates and Senate are at stake.

At that point, who knows?

Anything could happen. Maybe.

This article originally appeared on USA TODAY: 100 days until the Iowa caucuses. What could possibly go wrong?