An ABC News/Washington Post tracking poll showed the Republican leading his rival Hillary Clinton 46-45 percent, with news of a renewed FBI probe apparently devouring the Democrat's long-held lead.
Experts caution not to put too much stock in any one poll, especially one well within the statistical margin of error.
Clinton is still the overwhelming favorite, thanks to the quirks of the US electoral system, which tallies the winner based on weighted state-by-state races.
The New York Times' statistical model gives Clinton an 88 percent chance of winning, while respected website FiveThirtyEight says 74 percent.
But the shock ABC poll caps a series of surveys that point to a race which is narrowing in the final sprint.
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The idea that the 70-year-old Trump is still in with a chance will surprise many and prompt jitters among Democrats and the financial markets.
The bombastic reality TV star has been pummeled by allegations that he groped several women and has shady ties to Moscow.
A week ago pundits were rushing to declare the election over.
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Now, voices on Wall Street are warning that markets have not taken the prospect of a Trump presidency seriously enough.
"A Trump win could lead to a 11-13 percent sell-off," on the S&P 500 share index, analysts at Barclays warned clients Tuesday.
That would be a bigger crash than the day Lehman Brothers went bust in September 2008, the nadir of the financial crisis.
Source: AFP
Simon Johnson, former chief economist of the International Monetary Fund, warned such a shock "would cause the stock market to crash and plunge the world into recession."
The tumultuous two-year race -- featuring suggestions of sexual assault, Russian espionage and financial wheeler-dealing -- may have few twists and turns left in store.
On Monday Trump was hit by fresh allegations, revealed in The New York Times, that he not only dodged paying income tax, but did so in a legally dubious way.
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On the hustings
Clinton and Trump spent Tuesday barnstorming battleground states.
Trump -- somewhat out of character -- went to the oddly named town of King of Prussia in Pennsylvania to deliver a policy speech on healthcare.
"Our tax plan will provide a 35 percent tax cut to middle class families with two kids," he told supporters. "Our middle class has not been properly respected. That I can tell you."
Critics question how Trump would pay for his proposed cuts.
Meanwhile the Democratic nominee tried to steer attention away from her handling of US secrets and on to Trump's treatment of women.
The 69-year-old Clinton's level of support among women, who make up the majority bloc of voters, is as likely as anything else to decide who wins the election.
Her campaign spent days berating FBI director James Comey for revealing that the bureau is looking anew at her use of a private email server while secretary of state.
Clinton and her supporters were furious that Comey made his announcement without providing any new evidence of wrongdoing but, after three days of rage, Clinton took a more emollient tone Monday.
"I made a mistake. I'm not making any excuses," she said, inviting the FBI to pursue its probe and suggesting that the agency would find, as it did in July, that she has no case to answer.
"It wasn't even a close call," she said. "I think most people have moved on. They're looking and focused on 'OK, who is going to be the next president and the commander-in-chief?'"
With no sign anything concrete will come of the FBI probe before polling day, Clinton believes she can face down the challenge and return to the issue of Trump's fitness to lead.
'Trying to hold on'
Both candidates were keeping up a manic campaign schedule targeting key swing states.
Trump was scheduled to speak in Pennsylvania and Wisconsin on Tuesday, while Clinton was hopscotching across must-win Florida.
He has been warning supporters that a Clinton victory would mire America in "constitutional crisis."
He predicted "a criminal trial for a sitting president," and chastised Clinton for seeking to blame others for a scandal that has lasted for 20 months: "She has brought all of this on herself."
University of Virginia politics professor Larry Sabato said the FBI development has changed the race's dynamics.
"She would have been running a victory lap this week, running up the score," he told AFP. "Instead, she's trying to hold on."
But Sabato added that Trump's strategy of touring Democratic-leaning states reflects a stark truth: he needs to flip at least one of them in order to win, and even that may not be enough.
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