It is almost too late for an "October Surprise". But Democrat Barack Obama and Republican John McCain should be wary of the last weekend.
The final days of the last two presidential races featured unexpected twists that appeared to have an impact on the outcome, shifting the allegiances of some undecided voters in what turned out to be very tight races.
With the days counting down before the Presidential fight between Mr Obama and Mr McCain is decided on Tuesday, the chance for a game-changing moment that could reverse Mr Obama's lead over Mr McCain in national polls is dwindling but still alive.
"The last weekend can be a very weird and strange time in a presidential campaign," said Steven Schier, a political analyst at Carleton College in Minnesota.
"It's when the last of the voters who haven't been paying attention tune in, and the last of the undecided make up their minds. Strange things can happen," he said.
In 2004, al Qaeda leader Osama bin Laden injected himself into the campaign on the last Friday of the race with a new videotape. The tape reminded voters of the lingering risk from terrorism and President George W. Bush's efforts to keep the US safe.
Democrat John Kerry, embroiled in a tight battle with Mr Bush in most polls, saw the race slip away in the final days. He later blamed the tape for erasing his hopes of victory.
In 2000, the last wrinkle in the campaign broke on the Thursday night before the vote - Mr Bush, then the Texas governor, had been arrested in 1976 for driving under the influence and had not publicly disclosed the arrest.
Mr Bush campaign aides blamed the late disclosure for costing him what had been a slim lead in opinion polls over Democrat Al Gore down the stretch. Mr Bush narrowly lost the popular vote but won the Electoral College to claim the presidency after a disputed recount in Florida.
Exit polling indicated the revelation had played a role in denting Mr Bush's momentum in the final days of the campaign.
"Mr Gore clearly gained ground over the last few days of the campaign because of the DUI story," pollster John Zogby said.
So far, no thunderbolts have appeared in the race's final days to fulfil the US political tradition of "October surprises" like the collapse of hostage negotiations with Iran before President Jimmy Carter's 1980 loss to Ronald Reagan.
The phenomenon surfaced in 1968, when President Lyndon Johnson halted bombing of North Vietnam a week before the 1968 election. It was not enough to help his Vice President, Hubert Humphrey, beat Richard Nixon.
But the October surprise in this year's presidential race occurred in September - the economic crisis and Wall Street bailout that wiped out Mr McCain's edge in opinion polls and shifted the race towards Mr Obama.
The outbreak of a spiralling global economic crisis in the midst of a US Presidential campaign was unprecedented, and helped Mr Obama as polls showed voters favoured his leadership on economic issues.
There is little Mr McCain and Mr Obama can do to plan for any last-minute news events. Both will spend the weekend racing through battleground states like Ohio, Florida, Missouri and Colorado as Tuesday's voting draws near, with Mr Obama holding a lead in recent national polls of between 3 and 8 points.
A Fox News poll released on Thursday gave Obama a three-point edge, down from nine points last week. But six per cent remain undecided, and other polls show an even higher rate of undecided.
"At this late date there are still plenty of undecided voters who are about to make up their minds," Mr Schier said. "Late events can really determine the outcome of close races."