DENVER — President Obama and Mitt Romney confronted what one feared and the other hoped was an altered campaign on Thursday, pounding new urgency into what was shaping up as a wide-open final sprint to Election Day.

A day after the first debate, in which Mr. Obama was almost universally judged to have underperformed and Mr. Romney to have seized his opportunity, the president resolved to do what he did not do the night before: He went straight at the challenger, arguing forcefully that Mr. Romney’s moderate words masked extreme conservative policies.

“The man onstage last night, he does not want to be held accountable for the real Mitt Romney’s decisions and what he’s been saying for the last year,” the president said at a rally, looking more energetic than he was at his lectern Wednesday night.

Mr. Romney’s senior aides warned staff members and donors that the race was hardly won. But they said the debate reversed the sagging morale of volunteers and contributors and dispelled what had been a growing notion that the race was slipping away from Mr. Romney.