WASHINGTON — As the Obama campaign heads into its convention next week, Democrats see openings both to fill in unpopular details of Mitt Romney’s agenda left unsaid by Republicans in Tampa this week and to raise new questions about Mr. Romney’s character after widespread criticism of misstatements by him and his running mate, Paul D. Ryan.

“It’s sort of breathtaking that Paul Ryan made a whole speech about being truthful and making hard choices and yet he never mentioned a single idea of theirs — like turning Medicare into a voucher program, or a new $5 trillion tax cut they can’t pay for,” David Axelrod, President Obama’s chief strategist, said in an interview on Thursday, hours before Mr. Romney’s own acceptance speech. “They’ve spent an entire week not talking about their ideas because they know their ideas are unpopular.”

But worse than what Republicans have not said, Mr. Axelrod added, is what Mr. Romney and Mr. Ryan have said: “a compendium of demonstrable lies,” including widely refuted claims that Mr. Obama is trying to end work requirements for welfare recipients and is “raiding” $716 billion from Medicare beneficiaries.

“The audacity of mendacity,” Mr. Axelrod called it, in a play on the title of Mr. Obama’s 2006 book, “The Audacity of Hope.” He added, “It’s lying as a campaign strategy.”