WASHINGTON — History shows that candidates have different ways to score through presidential debates: the forceful put-down, the surprising show of skill, the opponent’s fumble, superior post-debate tactics.


But it also shows that to fundamentally alter the direction of a campaign, a candidate usually has to accomplish all of those things.


That underscores the challenge that Mitt Romney faces against President Obama as they approach the first presidential debate of 2012, the 27th of the television era featuring the major-party nominees.


In 2004, with Americans increasingly anxious about the Iraq war, Senator John Kerry knocked President George W. Bush onto the defensive by pointing out: “Saddam Hussein didn’t attack us. Osama bin Laden attacked us.” Mr. Kerry dented Mr. Bush’s lead, but ultimately could not overcome it.


In 1980, Ronald Reagan’s avuncular “There you go again” performance reassured Americans that he was not the extremist that President Jimmy Carter had warned about. Reagan’s standing improved after that debate, though the race had already tilted his way and a Gallup study later concluded that the debate was “not likely to have been a determining factor” in his landslide victory.


Four years before, President Gerald R. Ford blundered by asserting, “There is no Soviet domination of Eastern Europe.” Trailing Mr. Carter, the Democratic nominee, by double-digit margins before their three debates, Mr. Ford made up ground after the debates but went on to lose the popular vote by two percentage points.


Only twice have debates appeared to shift the election’s outcome. The first time was in 1960, when Americans first saw presidential candidates debate on television.


Senator John F. Kennedy, whose crisp, cool demeanor contrasted with Vice President Richard M. Nixon’s haggard appearance, moved from being even in the Gallup Poll to four percentage points ahead by the last debate, on Oct. 21. Gallup later concluded that the candidates’ four encounters that year “could very well have accounted” for Kennedy’s narrow victory, though the closeness of the contest and a dearth of other polling at the time make a definitive conclusion difficult.


The clearest shift from the debates came in the 2000 race, pitting Gov. George W. Bush of Texas against Vice President Al Gore. It resulted from a rare combination of factors, with devastating cumulative effects on Mr. Gore’s campaign.


Mr. Gore entered the first encounter, on Oct. 3, with a reputation as a strong debater and with a lead of five percentage points among likely voters in a New York Times/CBS News poll. “We weren’t all that far from where Romney is now,” Jan van Lohuizen, a pollster for Mr. Bush, recalled last week.


But Mr. Gore’s skill at jousting became overshadowed by minor factual misstatements and what appeared as a condescending, impatient demeanor — especially after Mr. Bush’s aides called attention to them in post-debate interviews.


“They beat us after the debate in the spin room,” said Tad Devine, a strategist for Mr. Gore. “Their spin was, ‘He lied and he sighed,’ and that took hold.”


It got worse when Mr. Bush’s running mate, Dick Cheney, bested Mr. Gore’s No. 2, Senator Joseph I. Lieberman of Connecticut, in the vice-presidential debate. In the second presidential face-off, Mr. Gore responded with what was widely judged to be an ineffectual performance.


Then, in their final debate, on Oct. 17, Mr. Gore overcompensated again — seeking to discomfit Mr. Bush by approaching him onstage. With a nod of greeting and an easy grin, Mr. Bush made Mr. Gore appear foolish.


Other errors by the Gore campaign during those two weeks, which included poor makeup for one debate that gave Mr. Gore an orange tint, helped Mr. Bush gain a strong edge in polls for “likability.” Daron Shaw, a political scientist at the University of Texas, called the result a “wave effect” that lifted the Republican ticket.