WASHINGTON >> History shows that candidates have different ways to score in 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 those things.

That underscores the challenge Mitt Romney faces approaching the first presidential debate of 2012 — the 27th of the television era — against President Barack Obama.

In 2004, with Americans increasingly anxious about the Iraq War, Sen. John Kerry knocked President George W. Bush onto the defensive by pointing out: “Saddam Hussein didn’t attack us. Osama bin Laden attacked us.” Kerry dented 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 concluded 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 Carter, the Democratic nominee, by double-digit margins before their three debates, 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.

Sen. 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 four encounters “could very well have accounted” for the Democrat’s narrow victory, though the closeness of the contest and dearth of other polling then makes a definitive conclusion difficult.

The clearest shift from debates came in the 2000 election, 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 Gore’s campaign.

Gore entered the first encounter, on Oct. 3, with a reputation as a strong debater and with a lead of 5 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 Bush, recalled last week.

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

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

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

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

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

The wave left Bush with a percentage point lead among likely voters in the post-debate New York Times/CBS poll — a net swing of 6 percentage points in the Republican’s direction. In surveys by NBC News/Wall Street Journal and Gallup, the swing was 9 and 12 percentage points, respectively.

If not for that shift, veterans of both campaigns say, Bush would not have gotten in position to eke out his Electoral College victory.

“The pivot point of the election,” Devine said. He still regrets the campaign’s inability to shape news coverage of the first debate, saying “we should have fought back harder” with a message that, if Gore sighed, “so did America.”

Even the most gifted political communicators have found debates an uneven terrain.

Reagan cemented his telegenic reputation by closing his lone 1980 confrontation with Carter with a question for voters: “Are you better off than you were four years ago?”

“We were headed for victory” anyway, said Ken Khachigian, who was Reagan’s speechwriter. But the strong performance “accelerated” Reagan’s momentum, he said, “maybe turning a very strong victory into a landslide.”

Four years later, Reagan’s Democratic challenger, Walter F. Mondale, gained the upper hand in their first debate. Steady and incisive, Mondale saw his poll ratings surge while Reagan, then 73, came across as fumbling and outmatched.

Reagan’s performance quickly triggered commentary — too much, in the Mondale campaign’s view — about whether he was too old to be president.

“That hurt us a lot,” Mondale’s press secretary, Maxine Isaacs, said by creating conditions for a backlash in the president’s favor.

Reagan made use of it in the second debate by declaring “I am not going to exploit, for political purposes, my opponent’s youth and inexperience.” Mondale, then 56, laughed along with viewers — and concluded his chance of erasing Reagan’s formidable lead had vanished.

“I said to myself, this is probably over now,” Mondale said.

He ultimately carried one state, his native Minnesota.

Mondale sees both possibility and risk in Romney’s quest to seize voters’ attention. The imperative for aggression collides with the need to show respect for a personally popular incumbent.

“These debates are the one chance to change how they look at him, and how they look at Obama,” Mondale said. The lesson of his own experience: “That’s a high hill to climb.”

© 2012 The New York Times Company