By Anthony Zurcher, the BBC's North America correspondent
Analysis: The US presidential election has been turned on its head now that it has been confirmed Donald Trump has tested positive for Covid-19.
That sentence could have been written about any number of moments in a tumultuous year in American politics, but nothing quite like this has occurred this year, this decade, this century.
Just 32 days before the presidential election, Trump has Covid-19. Given his age, 74, he is in a high-risk category for complications from the disease. At the very least, he will have to quarantine while he is treated, meaning the US presidential contest - at least his side of it - has been fundamentally altered.
Is the election campaign suspended?
The initial implications are obvious. The president's rigorous campaign schedule - which included visits to Minnesota, Pennsylvania, Virginia, Georgia, Florida and North Carolina in just the past week - is on indefinite hold.
Trump will certainly have surrogates on the trail, but given that he has relied heavily on his family and senior administration and campaign officials for such tasks in the past, and many of them may have to quarantine because of their own exposure to the virus, that operation will be disrupted as well.
Even the next presidential debate, a town hall format with audience questions scheduled for 15 October in Miami, Florida, is in doubt. Perhaps the event could be conducted via video-conference, but that will largely depend on the president's health at the time.
At this point, there is no serious talk of altering the election schedule, which would require an act of Congress passed by both the Democratic-controlled House of Representatives and the Republican-held Senate.
In some states, in fact, early voting has already begun.
How will this affect the polls?
Then there are the political implications. Despite the aforementioned turmoil this year - the pandemic and resulting economic disruption, the nationwide demonstrations against institutional racism and police brutality following George Floyd's death and sometimes violent unrest in several major US cities, the countless smaller crises and controversies that seem like a daily occurrence during the Trump years - this presidential race has been remarkably stable.
Democrat Joe Biden has held a statistically significant lead over the president for months in national polls, with a smaller but still noteworthy advantage in key swing states. Time was running out for the president to change this dynamic, even before this week's dramatic news.
The public has consistently given the president low marks for his handling of the coronavirus pandemic, so anything that puts the focus on the disease is potentially damaging for his re-election prospects. Complicating matters for the president will be that many Americans might recall what many would describe as the president's sometimes cavalier attitude toward Covid-19.
At the presidential debate on Wednesday, Trump belittled Biden for frequently wearing masks and not having campaign rallies that matched his own in size.
"I don't wear a mask like him," Trump said. "Every time you see him, he's got a mask."
While the president has, at times, stressed the importance of social distancing and taking the virus seriously, he has also trafficked in questionable science; said the virus would disappear "like magic"; and attacked state officials who have imposed more aggressive mitigation measures and been slower to reopen businesses and schools than he would like.
Trump's coronavirus infection will cast all of these past comments into sharp relief - once again raising questions about whether he took the pandemic seriously enough both on a national policy level and within the White House itself, where the president's health and safety must, for the nation's sake, be of paramount concern.
What are the risks for Democrats?
During times of national turmoil, the American public tends to rally in support of the president. While Trump and his administration will face hard questions about the virus, he and his wife will also be the recipients of national sympathy and prayers for the health ordeal that confronts them.
Democrats and the president's critics may be inclined to engage in a chorus of "I told you so" and celebrate what they see as political karma, but they do so at risk of seeming callous or indifferent to the crisis that confronts the nation.
More than 200,000 Americans have died of Covid-19 at this point - and grieving families and friends may not appreciate any form of victim-blaming.
The Biden campaign will face a challenge on how to respond. For months the Democratic nominee has kept a lower profile in order to limit his risk of exposure - and has been mocked by Republicans, including the president himself, for "hiding" in the basement of his Delaware home.
Recently, however, he has stepped up his in-person campaigning, including a train tour of Ohio and western Pennsylvania on Wednesday. The campaign also announced that it would resume door-to-door voter canvassing, although it had criticised the Trump campaign for conducting in-person activities in the past.
It seems unlikely Biden will suspend activities while the president is off the trail undergoing treatment, but his campaign may have to re-evaluate whether its recent activities should again be curtailed.
"Jill and I send our thoughts to President Trump and First Lady Melania Trump for a swift recovery," Biden tweeted. "We will continue to pray for the health and safety of the president and his family."
The Democratic nominee is setting a measured tone for his party - but it may be difficult for some to follow his lead.
Will this affect the Supreme Court nomination?
Trump's positive test is already sending shockwaves from the White House through Washington.
As Congress prepared to recess to allow its members to campaign for re-election, the legislature had been busy on two fronts - trying to negotiate a follow-up Covid-19 stimulus package, and begin the process of confirming Trump nominee Amy Coney Barrett to replace the late Ruth Bader Ginsburg on the US Supreme Court.
Trump administration officials, ones who have been in close proximity to the president, have been heavily involved in both undertakings.
Treasury Secretary Steve Mnuchin was at the Capitol yesterday negotiating with Speaker of the House Nancy Pelosi in a last-ditch effort to find agreement on relief legislation. And Barrett, along with White House chief of staff Mark Meadows and Vice-President Mike Pence, spent a long day on Wednesday visiting Republican members of Senate who will eventually vote on whether to seat her on the court.
Mnuchin, Meadows and Pence have all announced negative tests but none is fully in the clear yet.
The schedule for Barrett's pre-election confirmation was always going to be tight, requiring near-flawless execution in the face of Democratic efforts to disrupt and delay the process.
Now it seems almost certain that the vote will take place after November's vote, when there is the possibility that the American people would have dealt Trump and the Republican Party a defeat at the polls.
What about other consequences?
What other political knock-on effects could come from this news could depend largely on how far the virus has spread in the upper echelons of US government and how the president responds to his treatment.
The political uncertainty could further disrupt whatever economic recovery was under way, as public confidence plummets and businesses again brace for a drop in revenue.
Concerns about contracting the virus, sharpened by the president's diagnosis, could encourage more Americans to shift to mail-in balloting instead of in-person voting, causing delays in reporting the election results. If the election is close, the potential for a protracted legal fight over who won could increase.
In a year of political tempests, the biggest storms may be yet to come.