Elections are often described as a horse race but when Donald Trump seeks his second term in 12 months, it's going to look more like a monster car demolition derby.
One of the most divisive presidents in US history, fighting impeachment in a Congress paralyzed by partisanship, will go before an electorate split down the middle on November 3, 2020.
Democrats remain a long way from choosing Trump's opponent. The field of record size and diversity includes two African Americans, half a dozen women, an openly gay man and two top candidates seeking to move the United States dramatically to the left.
But one thing unites them: desire to humiliate former real estate tycoon Trump into becoming only the fourth one-term president since World War II.
One lead Democratic contender, centrist former vice president Joe Biden, says a "battle for the soul of America" is underway.
To which Trump says: Democrats want to "destroy our country."
And with US intelligence warning that Russia seeks to repeat its 2016 dirty tricks campaign, the possibility of the contest ending in genuine crisis cannot be discounted.
"You really have to stretch to find an election as combustible and unpredictable as this one," Allan Lichtman, distinguished professor of history at American University, said.
"There are very few analogues."
Battle of the bases
Trump's fate matters to the globe.
Trade disputes with China and the European Union, the future of NATO, US entanglements in the Middle East and Afghanistan, Iran's nuclear program, the North Korean nuclear standoff, the Paris climate accord — there's a near endless list of major issues that depend on whether or not Trump's "America First" agenda gets another four years.
But good luck to anyone trying to predict results.
Polls show Trump down against every serious Democratic candidate. His approval rate is stuck in the low 40 percent range, where it has been most of his presidency.
Yet this unpopular president shocked many experts when he defeated heavily favoured Hillary Clinton in 2016.
And polls this October by Moody's Analytics show he could repeat the feat, once again losing the popular vote but easily winning the all-important electoral college vote by focusing on strategically positioned strongholds.
Charles Franklin, director of the Marquette Law School Poll, says the depth and firmness of Republican-Democratic divisions nationwide mean there's little middle ground to play for.
"Support for the president is intense with his base and opposition is, if anything, more intense," he said.
"The group to watch," he said, is the small slice of voters who say they "somewhat approve" of the president. A shift to "somewhat disapprove" could be "a danger signal to the Trump campaign."
Impeachment wild card
Impeachment could yet throw all those considerations out the window.
Trump is accused by Democrats of withholding military aid to compel Ukraine to mount an embarrassing corruption probe against Biden — in other words using US foreign policy for his personal political benefit.
Trump dismisses the case as cooked up, but Congressional investigators have heard a steady flow of damaging evidence.
A vote to impeach in the Democratic-controlled lower house is seen as near inevitable. Trump would then face trial in the Republican-led Senate.
The conviction still seems highly unlikely. Yet even without being removed from office, a House vote to impeach would indelibly mark Trump's record.
Only two presidents have been successfully impeached before and at minimum, the scandal would fuel bitterness across the country, with unpredictable consequences.
Impeachment aside, the quantity of variables one year from voting day is already dizzying.
Will the Democrats nominate an unusually leftwing candidate like Elizabeth Warren, who wants to remake the US economy, or go for the less dynamic, but perhaps safer choice Biden?
Pete Buttigieg, a gay small city mayor from Indiana, was so unknown until recently that few could pronounce his name. (It's Boot-edge-edge). Today, he's climbing in the polls and seen as a potential dark horse.
And Trump, while not the unknown he was four years ago, remains wildly unpredictable, a convention-busting politician able to flip momentum with a tweet.
Last Sunday summed up his roller-coaster existence perfectly: bathing in the glow of victory with a morning announcement that US forces had killed the Islamic State leader in Syria, before getting booed at the World Series baseball game in Washington in the evening.
Finally, there are the so-called black swan events — things so crazy that they are hard to foresee, let alone prepare for.
One could stem from the expected renewed wave of Russian covert interference — a well-timed data dump, for example, or viral conspiracy theory or hacking of voting centers.
In some quarters there are fears that Trump could even refuse to accept defeat.
That sounds far-fetched, but the president has spent much of his first term claiming to be fighting off a mysterious deep state coup.
In the kind of nail-biting recount drama that saw Republican candidate George W Bush defeat Democrat Al Gore in 2000 thanks to a Supreme Court decision, the brawler Trump would be right in his element.
All of which adds up to making 2020 the most fraught contest for half a century, Lichtman says.
In 1968, Richard Nixon won after a traumatizing spell that saw Martin Luther King Jr and Robert F Kennedy assassinated, violent political protests, and race riots.
US institutions survived that turmoil. They also survived Nixon's subsequent resignation, just as they had survived the Civil War, the Great Depression and the civil rights struggle.
But will the country pull through again?
"That's an open question, isn't it?" Lichtman said.