Can McCain Pull Off October Surprise?

"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."