WASHINGTON -- Presidential debates have produced some of the most memorable moments of modern campaigns: A tanned and relaxed John F. Kennedy meeting a sweaty and pasty Richard M. Nixon in 1960. Gerald R. Ford denying Soviet domination of Eastern Europe in 1976. Al Gore sighing and rolling his eyes in 2000.
For all the lore and media buildup, the events haven't had much impact on election outcomes. "Where you started the debate season is pretty much where you end the debate season," said Temple University political science professor Christopher Wlezien, co-author of the book "The Timeline of Presidential Elections."
No candidate who was leading in the polls six weeks before the election has lost the popular vote since Thomas Dewey in 1948, according to Mr. Wlezien and Columbia University political science professor Robert Erikson. They studied polling data going back to 1952 and computed a running average "poll of polls" for each presidential election.
Mr. Gore, who had a slight lead over George W. Bush six weeks before the 2000 election, won a majority of votes cast in November, even though he lost the Electoral College tally that determines the presidency. The 1980 winner, Ronald Reagan, was tied six weeks before the election and pulled ahead of President Jimmy Carter before their only debate.
President Barack Obama, who debated Republican Mitt Romney on Wednesday night in Denver, was ahead 49 percent to 43 percent among likely voters in a Bloomberg National Poll conducted Sept. 21-24.
Mr. Wlezien and Mr. Erikson found only one campaign with a big movement in opinion polls from the start to finish of the debate series -- and then it was the candidate widely judged to have lost the debates who gained in the polls. Ford, notwithstanding his Eastern Europe gaffe, climbed 10 percentage points, narrowing the margin while still losing to Mr. Carter.
In 1968, when there were no presidential debates, Nixon's 15 percentage-point Gallup Poll lead in late September dwindled to a one-point win over Democrat Hubert Humphrey.
What influence debates have had on public opinion historically has stemmed from matters of style rather than substance. A glance at a watch or a distant reaction to an emotional question have been more consequential than clashes over war, taxes or economic policy.
A 2008 Gallup review of polling data surrounding presidential debates concluded that the events are "rarely game-changers," yet may have made a difference in 1960 and 2000 -- both among the closest presidential contests in U.S. history.
University of North Carolina political science professor James Stimson, author of "Tides of Consent: How Public Opinion Shapes American Politics," said it's not even clear that the debates were decisive in 1960 -- though Kennedy's cool, crisp on-air performance is often cited as pivotal to a campaign that marked the advent of the television age in politics.
"It's such a charming story," Mr. Stimson said. "You get the impression Kennedy was on the verge of losing when he debated Nixon. Instead, Kennedy was ahead going in."
Kennedy's average in the polls was 50.5 percent one week before the first debate. It was 50.6 percent one week after the last one, according to Mr. Wlezien and Mr. Erikson. They analyzed polling based on each candidate's share of the two-party vote, excluding independent and third-party candidates.
In 2000, an election decided by a few thousand votes in Florida, the debates "may have changed the outcome," said Tad Devine, an adviser to Mr. Gore. In the days after the first debate, press coverage focused on Mr. Gore's audible sighs and interruptions. Marked shifts in his demeanor during each of the next two debates and a faulty makeup job that gave him an unnatural hue in one renewed media questions about the authenticity of his public persona. "It changed the dynamic of the campaign," Mr. Devine said.
When Reagan debated Mr. Carter a week before the 1980 election, he was already in the lead. On average, polls taken one week before the debate showed the Republican with 51.7 percent of the vote, excluding support for independent candidate John Anderson and undecided voters, according to Mr. Wlezien and Mr. Erikson's compilation.
Even so, the Republican challenger disarmed Mr. Carter's attempts to portray him as an extremist, with an avuncular, head-shaking "There you go again." Undecided voters in focus groups that pollster Peter Hart convened for The Wall Street Journal said the debate relieved them of doubts about Reagan.
Debates such as the series between John Kerry and George W. Bush in 2004 have been more typical. Mr. Kerry attacked the incumbent's conduct of the Iraq War, reminding voters that "Saddam Hussein didn't attack us. Osama bin Laden attacked us."
"Kerry won three times in the debates and still lost the election," said Mr. Devine, who also advised Mr. Kerry.
In 1992, Democrat Bill Clinton's polling advantage declined slightly during the debate period, even though it was opponent George H.W. Bush who made one of the most often-cited gaffes of the campaign during their second encounter, looking down at his watch as the candidates fielded questions from voters in a town hall meeting format.
Michael Dukakis was almost 6 percentage points behind George H.W. Bush before the start of the 1988 debates, when his cold, unemotional answer to a hypothetical question about the rape and murder of his wife reinforced criticism of the Democrat as an aloof technocrat.
By the time candidates meet for debates, there aren't many undecided voters open to persuasion. Viewers mostly root for their favorite candidate and typically tell pollsters that their choice won.
By contrast, voter preferences have shifted decisively during the national political conventions, with the winning candidate taking the lead from his foe during the period in four of the past 15 presidential elections, according to Mr. Wlezien and Mr. Erikson.
Voter preferences aren't yet as entrenched during the conventions, and each candidate has an opportunity to make a sustained case for himself over several nights that typically dominate media coverage more than the debates, Mr. Wlezien said.