A day after his subdued debate performance, President Obama delivered a feistier critique of Republican nominee Mitt Romney, telling 12,000 supporters on Thursday that his rival had misrepresented his positions because “he does not want to be held accountable.”

Obama said the “very spirited fellow who claimed to be Mitt Romney” onstage at the University of Denver was an imposter who suddenly was dancing “around his positions” on tax cuts, education and outsourcing.

“It couldn’t have been the real Mitt Romney,” Obama told 12,300 supporters at Sloane’s Lake Park, “because the real Mitt Romney has been running around the country all year promising $5 trillion in tax cuts to the wealthy, but the fellow onstage last night did not know anything about that. The real Mitt Romney said we do not need any more teachers in the classroom, but the fellow onstage said he loves teachers, can’t get enough of them.”

Obama added that Romney misled viewers during the debate because he “knows full well that we don’t want what he’s been selling for the last year.”

The Obama campaign is hoping that by attacking Romney’s truthfulness, it can create a counter-narrative that while Romney might have won on style points, the president had offered more substance and consistency. The campaign has already produced a television ad with footage of Romney denying that his tax cut plan would cost $5 trillion and a tagline that says: “How can we trust Mitt Romney.”

Obama’s more direct and combative approach Thursday contrasted with what many observers felt was the lackluster effort he gave during the televised debate. Republicans quickly declared victory after the debate, and even some Democrats criticized the president for failing to confront Romney over his positions and statements.

Obama’s campaign aides began doing that early Thursday, making the rounds of morning cable television news shows and in a conference call with reporters. But they did acknowledged that it was not the president’s best performance and that changes could be made before the second presidential debate Oct. 16 in Hofstra, N.Y.

“We’re going to take a hard look at this,” senior campaign adviser David Axelrod said in a conference call with reporters. He added that the campaign will “have to make some judgments about where to draw the lines in these debates” and how to allocate its time during the answers.

Asked why the president had not attacked Romney on his time at Bain Capital and his dismissive “47 percent” remarks, Axelrod explained that the campaign did not think viewers wanted to hear Obama and Romney insulting each other.

At the same time, he continued to make the case that Romney had not been forthright with the public on several issues.

“He may win the Oscar for his performance last night, but he’s not going to win the presidency,” Axelrod insisted.

The Romney campaign scoffed at the criticism. Spokeswoman Amanda Henneberg said: “The Obama campaign’s conference call today was just like the president’s performance last night. The campaign, like the president, offered no defense of the president’s first term record or vision for a second term, and instead, offered nothing but false attacks, petulant statements and lies about Governor Romney’s record.”

At the outdoor rally, Obama followed up on Axelrod’s line of attack, saying “if you want to be president, you owe the American people the truth.”

The crowd appeared to appreciate the president’s directness. One man yelled out “Liar!” when the president said Romney was not coming clean on his record on outsourcing jobs while heading Bain Capital.

And Obama even delivered a zinger, mocking Romney’s statement during the debate that he would eliminate government funding for PBS, even though he likes Big Bird.

“Thank goodness someone is getting tough on Big Bird,” he said. “We didn’t know that Big Bird was driving the federal deficit.”

“And Elmo!” someone shouted.

“Elmo too,” the president replied.