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.