Millions of Americans will watch Wednesday night's highly anticipated U.S. presidential election debate, but analysts say that rarely in the last five decades have the encounters shifted the ultimate outcome of the race for a four-year term in the White House.

For days now, President Barack Obama, a Democrat, and his Republican challenger, former governor Mitt Romney, have huddled with advisors about their strategy, staged mock debates and pored over data on the sluggish U.S. economy and other domestic issues at the center of the first debate.

Within moments of its conclusion, news and political analysts will offer their assessment of each candidate's performance, looking for their memorable comments and misstatements of facts and suggesting how it might affect the November 6 election.

But political scientist John Sides of George Washington University in Washington and other analysts say that relatively few elections have been affected by presidential debates, notably in 1960, when the first of the debates in the television era was staged, and in 2000.

But Sides says — and pre-election polling of voters seems to confirm — that the 1960 and 2000 elections are an aberration in U.S. campaign history.

“Typically speaking, the debates aren't the driving factor in the election in any outcome … But in other races, even though the debates move the polls, they rarely do so to the extent that they turn the underdog into the frontrunner.”

He says, however, that does not mean that the Obama-Romney debate is devoid of importance.

“You know, if you believe the polls right now, we're looking at maybe at 5 to 6 percent of likely voters who report being undecided. So for the vast, vast, vast majority of people who are watching debates, they're really looking at voters who cheer on their own side. They've made up their minds and they're prepared to say that their candidate won the debate virtually no matter what happens. For that remaining sliver of people, it's possible that the debates can be influential. And here I would point to not just the role of the debate itself, but the role of news coverage after the fact in helping to interpret the debate and choose the winner and choose the loser.”