"There is a real sense that the future is not only unpredictable, but frightening. No one wants to think it or say it, but the kind of surprise McCain would need would have to be something terrible, something which played to McCain's perceived strengths and Obama's perceived weaknesses, something like a terrorist attack," said Smith.
The mere threat of terrorism created an October surprise in 2004, which observers say helped George W. Bush squeak past Sen. John Kerry in the final days of the campaign. Three years after 9/11, terrorism remained the number one issue in voters' minds that year, and when Osama bin Laden released an audio tape the last week in October in which he said, "any state that does not mess with our security, has naturally guaranteed its own security."
On Oct. 31, 2004, the polls put Bush and Kerry, D-Mass., in a dead heat. A week later, Bush won the election by 2.5 percent of the popular vote and netted eight more electoral votes than he won in 2000.
In 2000, Bush had to contend with his own October surprise, which did not actually occur until Nov. 2.
Five days before the 2000 election Bush admitted to a 1976 arrest for drunk driving near his family's summer home in Kennebunkport, Maine.
"The 2000 announcement didn't cost Bush the election, but it certainly cost him the margin. It could not have gotten any closer than it did," said Smith.
Debates are rarely the stage for a surprise revelation, Smith said, but sometimes, a particularly good performance can help swing the electorate. That happened, he said, in 1980.
The economy was the biggest issue of the 1980 election between Democrat incumbent Jimmy Carter and Republican nominee Ronald Reagan. Election Day that year came almost exactly a year to the day that 52 American diplomats were taken hostage at the American embassy in Tehran, Iran, by a group of Islamic extremists.
"The October surprise in 1980 was the debate," said Smith. "When you look at the economic problems today and the threat of terrorism, there is a close parallel with that election. The economic problems plus the hostage crisis merged and radicalized the electorate to something it wouldn't otherwise do -– elect an elderly, right-wing, former actor."
"Regan nailed it in those debates. Like this year, there was a sense that people were clearly dissatisfied by the status quo, but they were not yet convinced that Reagan was the answer. They needed to be convinced, and he did it in the debate. Reagan and Carter went from being essentially tied to Reagan winning by 10 points."
Perhaps the most iconic October surprise came neither in October, nor as a surprise. In the 1988 election between then-Republican Vice President George H. W. Bush and Mass. Gov. Michael Dukakis, Bush ran an ad depicting Willy Horton, a convicted felon sentenced to life, who committed rape and robbery while on a state-implemented weekend furlough in Massachusetts.
Though the connection between Dukakis and Horton was made in the primaries, Bush did not effectively use the information until his campaign put out a television ad called "Weekend Passes" on Sept. 21, 1988, that used Horton's image.
"There is something reminiscent in making that connection to an undesirable," said Clarke about the McCain strategy to connect Obama with Ayers.
"I'm betting [McCain] will try to do more of that. He'll keep raising issues and doubts about Obama's character and associates, including at the debate," she said.
"If it were anybody else but John McCain, I'd say this election is over. But time and again, he has proved his critics wrong. If anyone can pull this off, it is him."
And that might be the biggest surprise of all.