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 on Wednesday night in Denver, in the first debate against President 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 1976, President Gerald R. Ford blundered by asserting, "There is no Soviet domination of Eastern Europe." Trailing Democrat Jimmy Carter by double-digit margins before their three debates, Ford made up ground after the debates but not enough.

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

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 4 percentage points ahead by the last debate on Oct. 21.

The clearest shift from debates came in the 2000 election, pitting Texas Gov. George W. Bush 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. "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," Gore strategist Tad Devine said. "Their spin was, 'He lied and he sighed,' and that took hold."

In their final debate, on Oct. 17, Gore overcompensated -- seeking to discomfit Bush by approaching him onstage. With a nod of greeting and an easy grin, Bush made Gore appear foolish. "The pivot point of the election," Devine said.

Even the most gifted political communicators have found debates an uneven terrain. Reagan cemented his TV 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 Reagan speechwriter Ken Khachigian. But the strong performance "accelerated" Reagan's momentum.

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

Reagan's performance quickly triggered commentary about whether he was too old to be president. "That hurt us a lot," Mondale 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 lead had vanished.

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

He ultimately carried only his home state. 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."