The Obama campaign launched an attack against Mitt Romney’s business experience at Bain Capital on Monday, marking the beginning of what is almost certain to be a defining debate in the presidential campaign.

It was a moment that both sides have long expected, but one that nevertheless provided an early indication of how the battle over Bain — and the candidates’ differing versions of capitalism — will be fought.

In a two-minute ad that will air during the evening news Wednesday, the Obama campaign focuses on a shuttered steel mill in Kansas City, Mo., and the workers who lost their jobs after it went bankrupt under the supervision of Bain, a private-equity firm co-founded by Romney, now the likely GOP presidential nominee.

One former worker in the ad calls Bain a “vampire” that “came in and sucked the life out of us.”

The not-so-subtle theme — that Bain is a corporate menace that rewards investors at the expense of employees — mimics a strategy first used to devastating effect by then-Sen. Edward M. Kennedy (D-Mass.), who trounced Romney in a 1994 Senate race.

The Romney team responded swiftly. Within hours, it released its own 60-second video, titled “American Dream,” about Bain’s investment in Steel Dynamics, an Indiana company that Bain helped grow from 1,400 employees to more than 6,000. The campaign also distributed research to reporters and put senior adviser Ed Gillespie on a conference call with conservative bloggers.

The message from the Romney team was designed as both a defense and an attack: Obama just doesn’t understand how the economy works. Sometimes businesses succeed, sometimes they don’t — and the president would know that if he had spent any time in the private sector.

How the public comes to view Bain, a Boston-based company Romney led for 15 years, is critical to the former Massachusetts governor’s chances in November. He has pointed to his time at Bain and the business experience he gained there as the singular reason he is the right man to fix the nation’s troubled economy.

Bain was a pioneer in an aspect of capitalism that was widely accepted before the economic meltdown. It invests in start-ups, such as Staples, an office supplies superstore, and it makes leveraged buyouts of ailing companies that can lead to large-scale job losses through restructuring. Many Americans now consider such maneuvers part of the problem, a facet of finance that enriches investors while caring little for workers and their communities.

Democrats are trying to tap into that sentiment to raise doubts about Romney.

“That’s the heart of his campaign,” said Tad Devine, who helped craft similar Bain-related ads for Kennedy. “And if you undercut that, there aren’t a lot of places left for Romney to go.”

The two campaigns have been preparing for this moment for months: Obama began devising a Bain strategy over the winter, and the Romney team’s response was pulled from a library of prepared videos.

The Obama ad, which will air in Colorado, Iowa, Ohio, Pennsylvania and Virginia, highlights the story of a steel mill that Bain bought in 1993. GST Steel eventually went bankrupt, costing 750 workers their jobs, as Bain’s leaders walked away with an estimated $12 million in profit.

Obama also started a new Web page, , that shows a six-minute version of the video. The TV buy for Wednesday is not expected to be large, but advisers said the campaign will roll out more examples over the next several weeks as part of a broader strategy to portray Romney as a champion of the wealthy at the expense of the middle class.

Using Bain to make that argument was undercut somewhat on Monday by Steven Rattner, Obama’s former “car czar,” who said “the ad is unfair.”

“Mitt Romney made a mistake ever talking about the fact that he created 100,000 jobs,” Rattner said on MSNBC. “Bain Capital’s responsibility was not to create 100,000 jobs or some other number. It was to create profits for its investors. . . . I don’t think there’s anything Bain Capital did that they need to be embarrassed about.”

The Romney campaign pointed out Monday that the candidate no longer worked at Bain at the time of GST Steel’s 2001 bankruptcy; he had moved on two years earlier to lead the Salt Lake City Olympics.

The Obama team responded that Romney oversaw Bain’s purchase of GST and that he was still an officer earning profits when it went bankrupt.

Obama has his own connection to the private-equity industry. On Monday, he appeared at a fundraiser in Manhattan at the home of Hamilton “Tony” James, president of Blackstone Group, a large private-equity firm. Sixty people attended and donated $35,800 each.

This is the second go-round over Bain for Romney this year. During the Republican primary, challengers Newt Gingrich and Texas Gov. Rick Perry went after Romney’s record at the firm, with Perry famously referring to Romney as a “vulture capitalist.” Their arguments resonated less with Republican voters, many of whom dismissed them as an attack on capitalism.

Gingrich’s pollster, David Winston, said: “Newt’s attacking Romney over Bain didn’t work, and Newt’s moving off of it in South Carolina was a key decision in helping Newt win that primary.”

Romney responded at the time with a broad defense of free-market capitalism. And he argued that, overall, Bain helped create more jobs than the firm eliminated during his tenure there.

This time, some Republican strategists said Romney and his allies need to fight back quickly, recalling how devastating the Swift Boat Veterans for Truth attacks became for Sen. John F. Kerry (D) in the 2004 race after he waited too long to fully respond.

“I think the campaign and its surrogates and the outside groups need to get out the real story about his Bain record,” said Rick Reed, who helped produce the Swift Boat ads and worked for Romney’s 1994 campaign.

But Romney advisers said they have no interest in litigating the specifics of the GST Steel bankruptcy or any other examples the Obama campaign may highlight. Instead, they are trying to use the president’s attack as an opening for a broader debate over the two candidates’ economic credentials, believing that Romney’s business know-how is his chief qualification for the presidency.

“Every business entrepreneur knows that you have successes and failures, and in Mitt Romney’s case, he’s had many more successes than failures,” said Eric Fehrnstrom, a senior adviser to the candidate.