Anthony Zurcher – BBC North America reporter
In just under five months Democratic presidential nominee Hillary Clinton may be sleeping soundly in the White House. Before election day arrives on 8 November, however, she may be tossing and turning in bed, wondering if there’s something out there that could derail her presidential ambitions.
While the former first lady has a poll lead that ranges from dead heat to slightly ahead and an electoral playing field that gives her a decided advantage, Republican nominee Donald Trump – despite an up-and-down past month – remains in striking distance.
Whether this is a reflection of Mrs Clinton’s weaknesses as a candidate or Mr Trump’s unconventional appeal, the reality is the race could still tip towards the Republican if something breaks his way.
Here are five reasons why Mrs Clinton could suffer from election-related insomnia despite conventional wisdom that has favoured her for months.
There are the things a candidate can control – the message, the on-the-ground organising, the advertising campaigns. Then there are the “black swans” – unpredictable on-off events that can render all of those preparations irrelevant.
If Mrs Clinton occasionally wakes in a cold sweat between now and November, it could be because she’s afraid of something totally out of her control.
Given that Mrs Clinton’s poll standings have closely tracked American confidence in the economy, a financial collapse could prove devastating to her candidacy.
A terrorist attack on US soil could also turn the race on its head. Although Mr Trump is widely viewed as mishandling his response to the Orlando shootings in June, the November attacks in Paris proved a boon to his primary campaign. A high-profile incident could make many Americans take another look at those who would be their commander in chief.
Then there are natural disasters and the competency of the ensuing government response. Hurricane Sandy helped boost President Barack Obama’s standing in 2012, while Hurricane Katrina devastated the Republicans in 2005. If tragedy strikes, there’s no telling how the politics of the situation could play out.
Hillary’s fear: As any horror movie aficionado knows, the scariest monster is the one that’s unseen. An “October surprise” on the eve of an election is the under-the-bed bogeyman that every politician dreads.
Ms Clinton has seen her ratings for honesty and trustworthiness damaged by stories relating to donations by foreign actors to the Clinton Foundation and her use of a private email server while she was secretary of state.
Although the information that has emerged has been embarrassing, and the FBI investigation into her email practices paint an at-times-unflattering picture, they have not presented a mortal threat to her candidacy… yet.
But what if damning evidence surfaces – a “smoking gun” that even her supporters can’t explain away? There’s currently a batch of 17,000 emails the FBI recovered from Mrs Clinton’s server – documents not originally handed over to the government by Clinton personnel – that is being reviewed by the state department and will probably be made public by the end of October.
Then there’s the possibility of more hacked Democratic communications being published by WikiLeaks. The group’s founder, Julian Assange, has already hinted that Clinton-related documents could come out before the election.
It’s all a big question mark. Political candidates hate question marks.
Hillary’s fear: Smoking guns are popular in murder mysteries, but they rarely happen in big-time politics. Mrs Clinton’s opponents have spent the better part of 30 years supposedly on the verge of uncovering that key bit of incriminating information, but it never seems to turn up. Only Mrs Clinton knows if her luck is about to run out.
Following the Democratic National Convention in late July, Mrs Clinton surged to a modest but steady lead in national polls and a more convincing advantage in key battleground states.
Now, however, Mr Trump appears to be cutting into her advantage. Many polls show the race tightening, and a few give the Republican nominee a slight lead.
If the numbers stay close heading into election day, there’s the possibility that the polls could be understating Mr Trump’s support enough for him to end up on top. For instance current survey models could be downplaying the turnout of white male voters – a key demographic for the Republican candidate.
There’s also the possibility that respondents are unwilling or embarrassed to tell pollsters they support Trump – an American version of the “shy Tory” phenomenon that powered John Major to re-election as British prime minister in 1992 (and may have resurfaced in 2015’s general election and the Brexit vote).
Hillary’s fear: Four years ago Republicans, from presidential nominee Mitt Romney on down, believed the election eve polls were biased against them. It turned out the numbers were largely spot on, and Mr Obama cruised to re-election. Since then, however, there have been some glaring misses – in Canada, the UK and several US primary races. Much to Mrs Clinton’s dismay, off-kilter polls would be far from the most surprising development in this presidential election season.
Mr Trump made headlines in August when he warned that the election could be rigged against him. While his fears centred on vague allegations of potential voter fraud, Mrs Clinton has a different source for her election-day concerns – nefarious digital saboteurs.
While talking to reporters on her campaign plane on Sunday, she compared recently hacked Democratic committee emails to a modern-day Watergate break-in. The risk to the US electoral system could be greater than just purloined communications, however.
The FBI has found evidence that hackers have breached state-based electoral databases in Illinois and Arizona. It issued a warning to US election officials to upgrade their security protocols and be vigilant against future cyber-attacks.
Although it would be extremely difficult for a hostile actor to surreptitiously alter the outcome of a US presidential election, even a handful of questionable results in key precincts could be enough to cast the entire election results in doubt.
Hillary’s fear: Given the patchwork nature of US electoral systems, each managed by state and local officials, ensuring airtight security is nearly impossible. Against a determined foe, there may be no way to prevent an electoral doomsday scenario that turns a clear-cut Clinton victory into a legal morass. That’s not just a Clinton nightmare scenario, it’s a national one.
The three presidential debates, the first of which is scheduled for 26 September, are a huge opportunity for Mrs Clinton to draw contrasts with Mr Trump on knowledge and experience – but they also have the potential for disaster.
The audience for these debates will likely be huge, rivalling the 70 million who tuned in for the highly anticipated vice-presidential showdown between Joe Biden and Sarah Palin in 2008 – or even the record-setting 80 million for the 1980 presidential debate featuring Ronald Reagan, Jimmy Carter and independent candidate John Anderson.
Mr Trump is an unconventional debater, but as he demonstrated in 11 Republican primary face-offs, he can be a wily opponent. While he was often thin on details, he had a showman’s skill on the stage and kept his opponents off balance.
As the unpolished outsider, Mr Trump will enter the debates with lower expectations than career politician Clinton. If he acquits himself well, he could convince reluctant voters to come over to his side.
Hillary’s fear: John Weaver, Ohio Governor John Kasich’s campaign strategist, famously compared the Republican primary debates to a car race where one driver was intoxicated. For Mrs Clinton the presidential showdowns will be more like a boxing match with late-career Mike Tyson. She may feel confident in the ring with Mr Trump, but there’s no telling what he might do.