Story highlights Obama campaign and Democratic National Committee are in Tampa getting their message out

L.A. mayor: "It's worth asking: What did Romney build?"

Maryland governor: "Romney economics would spell disaster for America's middle class"

Romney camp fires back: Obama "cannot defend a record of broken promises and failed policies"

Democrats wasted no time on Tuesday trying to undercut Republicans as they began their convention, attacking a key GOP message before party luminaries took the spotlight in Tampa.

Republicans aggressively pushed their convention theme, "We Built It," while Democrats sought to turn it back on Mitt Romney, who was affirmed as the nominee by a roll call of delegates in the first significant moments of the event.

Republicans highlighted a remark by President Barack Obama, who said in July that public investment in infrastructure helps businesses grow. "If you are successful, somebody along the line gave you some help. ... If you've got a business, you didn't build that," Obama said at the time.

Republicans have seized on the comment to suggest that Obama is out of touch with small business owners and does not understand how the private sector works. Democrats, however, contend that their rivals are taking things out of context.

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"So as Republicans continue to distort the president's words and push their 'We built that' theme in Tampa," Los Angeles Mayor Antonio Villaraigosa said from the "war room" Democrats have set up in downtown Tampa near the convention site. "It's worth asking: What did [Mitt] Romney build?"

Villaraigosa, chair of next week's Democratic convention in Charlotte, answered his own question - "A bank account in Switzerland, investments in Bermuda and the Caymans, and an inexplicably large IRA. That's what he built."

Democrats are trying to nail Romney with a one-two punch: casting him as wealthy and privileged and saying that his policies would hurt the middle class.

"A stronger, growing middle class means a stronger, growing American economy," Maryland Gov. Martin O'Malley said, "And that's really what this election is all about." "On a national scale, Romney economics would spell disaster for America's middle class, would be wrong for America's middle class, would not grow our middle class. And, in fact, would harm our middle class," O'Malley added.

On Romney's tenure at the top of Boston private equity firm Bain Capital, O'Malley said the Republican's goal when investing in troubled companies was not creating jobs. Instead, O'Malley suggested that the firm under Romney destroyed businesses and jobs in order to generate profit for investors.

"In this economy, there are shipbuilders and there are ship-wreckers," O'Malley said. "There are those who manufacture automobiles and there are those who scrap automobiles. There are those who build up and create things and there are those who make a quick profit by dismantling things."

The Romney campaign quickly fired back.

In an email, Romney spokesman Ryan Williams accused Democrats of launching "false and baseless attacks" against the former Massachusetts governor whose tenure at the firm prior to his public life made him wealthy.

"The facts speak for themselves," Williams said, "with 23 million Americans struggling for work, nearly one in six Americans living in poverty, and median incomes declining, the Obama campaign cannot defend a record of broken promises and failed policies. Mitt Romney and Paul Ryan have a plan to strengthen the middle class by creating jobs and turning around our economy."

Asked at the event about recent polling indicating the public views Romney stronger on handling the economy, Obama senior adviser Robert Gibbs countered. Gibbs pointed to other recent polling data suggesting the public is more partial to Obama on the questions of who cares about people and who cares about the middle class.

"I think those polls demonstrate overwhelmingly that the president is their pick," Gibbs said.

The event was the latest salvo in an escalating pace of aggressive campaigning, rhetoric, and attacks on the part of both presidential campaigns.

In a break from tradition , the president is out campaigning this week while Republicans hold their nominating convention. Speaking in Iowa Tuesday, Obama said he expects the event to be a "pretty entertaining show."

"I'm sure they'll, you know, have some wonderful things to say about me," he joked.