Beside the olive display at Zabar’s, that iconic hub of lox and neurosis on the Upper West Side of Manhattan, Linda Donohue was trying to talk herself down.
“We have to have more faith in the American public,” said Ms. Donohue, 61, a longtime New Yorker now living in Seattle.
A man behind her could not suppress a loud snort.
Then Cathi Anderson, who was shopping with Ms. Donohue, mentioned yet another distressing poll, this one from Ohio, which showed Mr. Trump ahead. Ms. Donohue nodded grimly.
Just in case her faith in the American electorate was misplaced, Ms. Donohue said, she had retained her Irish citizenship.
For both parties, every election can feel like the most vital of a lifetime, the one day standing between a still-proud nation and its imminent demise. Among liberals, there is an especially rich tradition of “bed-wetting,” as even some practitioners call it, at the faintest sign of shakiness from their candidate.
But as Hillary Clinton lurches toward Election Day, her supporters at times seem overwhelmed by a tsunami of unease, exacerbated by Mrs. Clinton’s bout of pneumonia and a slow-footed acknowledgment of the illness. They are confronting a question they had assumed, just a few weeks ago, they would not need to consider in a race against the most unpopular presidential nominee in modern times: Could Mrs. Clinton actually blow this?
“It’s like someone dropped ice water on the head of America,” Julie Gaines, the owner of Fishs Eddy, a home goods store in Manhattan, said of Mr. Trump’s increased odds. “Everyone sobered up. This could happen.”
The creeping dread has accelerated in recent days, reaching critical levels even by Democratic standards.
Mrs. Clinton became sick. Several polls tightened to the margin of panic, with Mr. Trump overtaking her in surveys in Ohio and Florida. And even as Democrats hoped on Friday that Mr. Trump’s latest gambit — seeking to distance himself from his long history of “birtherism” — would backfire, there is a fear that no scandal can sink him.
A cartoon in The New Yorker captured it best: A woman sits in her psychiatrist’s office, perspiring in distress. The doctor scribbles on a pad. “I’m giving you something for Hillary’s pneumonia,” the caption reads.
Supporters of Mrs. Clinton have greeted the moment with varying degrees of alarm, according to interviews with dozens of them across the country.
They read warily about the health of her lungs and her swing-state field operations. They reassure one another by reminding themselves of President Obama’s two winning campaigns, which encountered similar fits of concern after Labor Day.
But even some zealous Clinton defenders have grown frustrated with their candidate, marveling at the prospect of her snatching defeat from the jaws of victory, for which some say they would never forgive her. The campaign’s decision last week not to acknowledge Mrs. Clinton’s pneumonia until two days after a diagnosis, once video surfaced of her stumbling out of a Sept. 11 memorial service on Sunday, has especially rankled.
“They kept it from us,” said Sonia Ascher, 74, a former campaign volunteer, sitting with her husband and son at a coffee shop in Portsmouth, N.H. “It was just another thing again, another mistake, which she really can’t afford right now.”
The gloom seems to be spreading. Maurice Doucet, 55, a software engineer from Portland, Ore., wondered aloud on Wednesday how the race had gotten this close, lamenting Mrs. Clinton’s use of a private email server as secretary of state.
“The rational side of my brain goes, ‘There’s no way people are actually going to switch sides,’” he said of voters’ movement toward Mr. Trump. “But the emotional side,” he added, his voice trailing off moments later.
In Washington Square Park in Manhattan, among smitten college students and acoustic guitars, Guillermo Vidal, 75, grimaced at the thought of a Trump presidency. His friend’s bichon frisé-poodle mix, Dipsy, sat on the bench beside him, looking fretful.
Mr. Vidal was asked what was troubling Dipsy.
“She’s a Democrat,” Mr. Vidal said dryly.
Then there are those who traffic in angst professionally, who are straining to consider even the chance of a Trump election.
“The possibility of that is too horrifying to broach,” Larry David, the “Seinfeld” co-creator and “Curb Your Enthusiasm” star, wrote in an email. “It’s like contemplating your own death.”
Surely Mr. David had done that, too?
“I can’t go there,” he said.
In liberal enclaves, some modest contingency planning has begun. Threats of relocation are a bipartisan ritual every four years, expanding the audience for Canadian home listings. But this time, voters seem to be taking their research a bit more seriously.
Ramona Gant, 28, a graduate student in Chicago, said she had just renewed her passport with the election in mind.
Ms. Donohue’s friend at Zabar’s, Ms. Anderson, also of Seattle, mused that Vancouver was not too far up the road.
Mike Brennan, 67, from Ventura County, Calif., is keeping an eye on the stock market.
“If it looks like it’s going to be close,” said Mr. Brennan, a Republican supporting Mrs. Clinton, “I’ll pull my money out.”
For many Americans, Mr. Trump’s momentum has registered as a more visceral threat, heightening concerns that had festered since his candidacy began. Ahtziry Barrera, 18, a college freshman from Orlando, Fla., arrived in the United States more than a decade ago from Mexico. Under an Obama administration policy known as DACA, which Mrs. Clinton has vowed to protect, Ms. Barrera has been allowed to stay in the country because she entered as a child.
“A lot of people were not taking him seriously. ‘Oh, he’s not going to win.’ But it happened,” she said of Mr. Trump’s success in the Republican primary race. “It is a big stress on me, knowing that Nov. 8 is the day I’ll know whether my future is going to be secure or not.”
Some optimists have preached calm, reminding one another of Mrs. Clinton’s organizational advantages and holding out hope that she can best Mr. Trump decisively on a debate stage.
While allowing that Mrs. Clinton has not run a perfect race, many of her admirers have cast blame elsewhere, singling out the news media, the Republicans who nominated Mr. Trump and, of course, the man himself.
Gloria Steinem, the feminist leader and a Clinton supporter, said in an email that she had sensed a growing worry in recent weeks, fearing that Mr. Trump’s candidacy was becoming “legitimized by ‘media evenhandedness’” that had made his assorted scandals seem more banal.
“There are things any campaign could do better, but in this case, I fear it’s like blaming the victim instead of the bully — dissociating in the hope that we won’t be bullied, too,” she wrote. “It’s like saying, ‘If only she hadn’t been walking in that neighborhood. …’”
Others hold tight to an abiding belief, perhaps against their better judgment, that everything will work out.
“I still believe in humanity,” said Nadia Johnson, 22, a Brooklyn resident lunching at a Whole Foods this week.
She quickly added a request: Ask her again in November.