DENVER — President Obama and his team woke up here on Thursday morning confronted by the realization that he lost his first debate by passively letting Mitt Romney control the conversation. Then the president and his advisers resolved to do what he himself did not the night before.


Under fire from fellow Democrats, Mr. Obama came out swinging, accusing Mr. Romney of lying to the American people about his plans for the nation. “I met this very spirited fellow who claimed to be Mitt Romney,” Mr. Obama told 12,000 supporters during a lakeside rally. “But it couldn’t be Mitt Romney, because the real Mitt Romney has been running around the country for the last year promising $5 trillion in tax cuts that favor the wealthy. The fellow onstage last night said he didn’t know anything about that.”


He said the Mr. Romney of the debate wanted to put more teachers in classrooms and claimed not to know companies get tax breaks for outsourcing jobs. “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. “And that’s because he knows full well that we don’t want what he’s been selling for the last year.”


The vigorous assault on Mr. Romney suggested just how worried Mr. Obama’s campaign has become. The president’s advisers concluded that he had lost his first debate by not pressing Mr. Romney enough. After a series of late-night and early-morning consultations, the Obama team decided to try to correct that Thursday with a more aggressive stance, including the rally rhetoric, a new television ad and a conference call questioning Mr. Romney’s truthfulness.


David Axelrod, the president’s strategist, called Mr. Romney an “artful dodger” whose debate comments were “devoid of honesty,” “rooted in deception,” “untethered to the truth” and “well delivered but fraudulent.”


“Not surprisingly, what we learned is he’ll say anything,” Mr. Axelrod said. “That makes him effective in the short term but vulnerable in the long term.” He added, “He may win the Oscar for his performance last night but he’s not going to win the presidency.”


   The Romney team, feeling rejuvenated, fired back. “In full damage-control mode, President Obama today offered no defense of his record and no vision for the future,” said Ryan Williams, a Romney spokesman. “Rather than a plan to fix our economy, President Obama simply offered more false attacks and renewed his call for job-killing tax hikes.”


In trying to turn the tables on Mr. Romney, the president’s team was hoping to salvage a debate performance widely criticized by Democrats and Republicans alike. Aides described Mr. Obama as out of practice at debating and said he made a conscious decision not to bring up some of the campaign’s favorite attack lines of recent months, a decision they left little doubt disappointed them.


Now they will have to make what Mr. Axelrod called “adjustments” in the president’s approach for the next debate on Oct. 16. The “take-away from this debate,” he said, was that they “can’t allow someone to stand there and manhandle the truth.”


Mr. Obama’s advisers appeared almost to expect a different Mitt Romney to turn up for the debate: the hard-edged conservative who had largely pitched his message to the Republican base. Instead, Mr. Romney softened his rhetoric, promising that his reform of Medicare would not touch benefits for older Americans and praising elements of Mr. Obama’s education policy. He also tried to paint the president as a protector of big banks because of the post-financial-crisis government bailout of the banking industry.


Campaign officials said they wished Mr. Obama had called out Mr. Romney on assertions that they said were untrue, although they conceded that some of the weaknesses in the president’s performance were simply part of his approach to debates, which is to shy away from highly personal confrontations. The base wants him “to gut Romney,” one adviser said, but swing voters hate that and the president was trying to find a balance.


Even so, Democrats questioned why he did not bring up a range of issues they considered favorable to Mr. Obama, including women’s rights; Mr. Romney’s taxes; the Republican candidate’s comments about the “47 percent” of Americans who consider themselves “victims” dependent on the government; and the record of his former firm, Bain Capital. Some Colorado Democrats leaving the debate seemed despondent at the president’s performance and fretted that he had put their state back in play.