Threats both foreign and domestic are putting the United States on edge, as an election campaign that has stoked tensions like no other draws to a close.
In New York, Virginia and Texas, authorities have been alerted by federal intelligence services to a heightened threat of terror attacks by al-Qaeda for the day before the election. Security officials are also making known their concern about cyberattacks – particularly, though not exclusively, from Russia – disrupting or undermining Tuesday's vote.
And meanwhile, cities with large African-American populations are nervously waiting to see whether Donald Trump's call for supporters to monitor "urban" areas, which he claims without evidence are rife with voting fraud, will lead to election-day harassment or violence.
But for many Americans, and indeed much of the Western world, what is most disconcerting is the sudden sense of uncertainty about the election's outcome and aftermath. Until little more than a week ago, Mr. Trump was suffering so much in the polls – following the release of a video in which he boasted about sexually assaulting women – that few people were giving serious thought to what his presidency would look like. And Hillary Clinton appeared headed for a big enough victory that Mr. Trump would have trouble sowing doubts about its legitimacy, as he threatened by refusing to promise he would accept the election's result.
Now, a terrible spell for Ms. Clinton – mostly because of the return of the controversy around her e-mail practices while secretary of state, and untimely news about rising Obamacare premiums – has allowed an uncharacteristically disciplined Mr. Trump to close the gap nationally. And polls show him within striking distance in most battleground states that will decide the result.
By any normal logic, Ms. Clinton should still win. There are more large states solidly in the Democratic nominee's favour than the Republican's, so Mr. Trump needs to almost run the table in the battlegrounds. Ms. Clinton has a much more sophisticated get-out-the-vote operation than does Mr. Trump. Advance voting numbers suggest that while black turnout is down from Barack Obama's election victories, turnout among Hispanics is up. And public-opinion experts point out that the scale of Ms. Clinton's previous lead and the closeness now have probably both been exaggerated – the result of candidates' supporters being more likely to participate in polls when the news cycle favours their candidate.
But with two historically unpopular candidates competing in a uniquely off-putting campaign, nobody can be certain normal logic applies. Never before has a candidate relied as much as Ms. Clinton on voters being motivated to come out in opposition to her opponent rather than out of enthusiasm for her. Mr. Trump is unusually dependent on antipathy toward his rival, too, but also on an unusually motivated white, working-class base that could vote in higher numbers than is the norm. And shifting demographics – rapidly growing minority populations in states Republicans usually win, aging white populations that jeopardize the Democrats' hold on others – are turning the electoral map on its head. For the campaigns themselves, there should be less uncertainty than among the general public, because their internal voter tracking is much more sophisticated than media polling. But the way Ms. Clinton is responding to whatever information is at her disposal does not exactly offer reassurance for those terrified by a Trump presidency. Until recently, she was trying to expand her map by appearing in states Democrats don't normally win; now, she's turning up in places like Michigan, which are supposed to be locks. And she has resumed advertising in states where she had stopped because they seemed safe.
If Ms. Clinton does hang on, as still remains the bet, the odds of a smooth transition after election day are not improving. Mr. Trump's recent momentum may help persuade his supporters that a defeat is proof the election is "rigged," as he has claimed. And the spectre of Ms. Clinton being under FBI investigation will affect her perceived legitimacy.
By all appearances, Mr. Trump will mount legal challenges if he loses narrowly, with close margins in key states. If so, there is a slim but not completely discountable chance that the result will wind up with a Supreme Court currently deadlocked between liberals and conservatives, because Republicans blocked Mr. Obama from filling its vacancy.
According to a poll published on Friday by the New York Times and CBS News, more than four in five voters have been "disgusted" by the race. For some, the prospect of the campaign not actually ending on Tuesday may be most terrifying of all.