MADISON, Wis. — His performance panned, President Barack Obama is changing his debate strategy against Republican Mitt Romney, aides conceding the president must find a crisper way to sell his agenda and counter his opponent without getting lost in the weeds.

The heart of Obama's new message with less than five weeks to go: Romney is a liar.

Expect that theme — expressed in softer terms from the president than from his aides — to drive Obama's advertising and messaging for days. Wednesday night's debate showed Obama was rusty, rambling and cautious, but his aides insist he emerged with a real opening to target Romney's assertions.

"Gov. Romney may dance around his positions, but if you want to be president, you owe the American people the truth," Obama declared in his first post-debate appearance, a Thursday rally in Denver. He displayed an energy that was conspicuously absent in the debate.

The new line of argument is based on the Obama campaign's contention that Romney, while sharp and commanding on the debate stage, delivered a series of statements that don't stand up to factual scrutiny. They singled out Romney's positions on tax cuts, education and outsourcing as misleading to the middle class.

David Plouffe, the Obama White House adviser who ran his 2008 campaign, called Romney's performance "probably unprecedented in its dishonesty."

Obama's campaign quickly released an ad raising questions about Romney's honesty, arguing that he didn't level with middle-class families on how his tax plan would affect them. "If we can't trust him here, how could we ever trust him here?" the ad says.

It was airing in Colorado, Florida, Iowa, New Hampshire, Nevada, Ohio and Virginia. Separately, a campaign official said Obama's team had its strongest fundraising month of the 2012 election cycle in September, exceeding August's haul of $114 million. The official would not say how much the campaign raised and requested anonymity because he or she was not authorized to discuss the fundraising publicly.

At the same time, the Obama camp was forced into its own difficult appraisal of the president's performance, with no shortage of critical outside opinions, either.

Those close to Obama said he was so intent on answering questions and not letting Romney rile him that he came across as wonky and lacking punch.

"Obviously, moving forward, we're going to take a hard look at this, and we're going to have to make some judgments as to where to draw the line in these debates and how to use our time," said David Axelrod, the Obama campaign's senior adviser. "I'm sure that we will make adjustments."

Plouffe put it this way when asked about those adjustments: "We just need to account for Romney's dishonesty."

But Obama had other problems, driven in part by a debate format that does not play to his strengths.

He did draw distinctions with Romney on a host of issues central to the campaign, but often did so by seeming to talk to moderator Jim Lehrer more than the audience or the man trying to take his job.

Obama aides acknowledged that the president delved more deeply into the intricate details on policy than they had planned and fell into one of the patterns they had most hoped to avoid — long-winded answers that lacked a clear emotional connection with voters.

Coming out of the debate, in his advertising and his speeches, Obama started hammering Romney for a lack of specifics on his tax plan, and for not being clear about how he would replace the president's health care law and Wall Street regulations.

But the Democratic campaign said there would be no wholesale changes to its strategy.

What's more, Obama aides said it was not at all clear that Romney's debate win would translate where it matters — into votes.

Axelrod said the immediate data gathered from voters showed they gave Romney the edge in performance but broke evenly on how it influenced their vote.

Plouffe said the real measure for Romney was this: "Is he going to take the lead in Ohio? If he doesn't, he's not going to be president."

The next presidential debate is in New York on Oct. 16, followed by a final one in Florida on Oct. 22.