WASHINGTON — President Obama’s re-election campaign opened up its Bain Capital-focused line of attack on Mitt Romney this week, releasing a television ad, web video and website hammering the presumptive Republican nominee’s work at the venture capital firm.

At the heart of the new ad is the tale of GST Steel, a Missouri steel mill that Bain purchased in 1993 and that went bankrupt in 2001. David Foster, a Minnesota Democratic activist who was a steelworkers union leader at the time of the sale, appears in the ad.

Foster was the regional director of a steel workers union when Bain purchased GST Steel, and he was a lead negotiator for workers during the Bain years. In an interview, Foster echoed the ad’s central message — that Bain, led by Romney, acted as a “financial predator” that swooped into GST Steel, saddled it with debt, cut benefits for employees, reaped a healthy profit for its investors and let the company fall into bankruptcy in the end. Romney left Bain in 1999 to lead the Olympics in Salt Lake City.

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As the company went bankrupt, Foster was the one who broke the news to its employees, telling them they were losing health insurance, vacation time and chunks of their pensions. In the ad, he called it “among the most painful experiences of my life.”

The Obama campaign’s ad will run only once (during the evening news tonight), in only five markets (none in Minnesota). It’s a very small ad buy — only $83,000, reportedly, though the ads have already been viewed more than 400,000 views online — but it represents the Obama campaign’s first salvo in what is likely to be a lengthy and bitter fight to define who Mitt Romney is and what he stands for.

The Romney campaign was ready for the offensive. After the Obama ad was announced on Monday, it released its own video about how “the American Dream” played out a different steel plant, Steel Dynamics, which quickly grew after an initial round of investments from Bain.

The campaign also highlighted a statement from Steven Rattner, a former Obama adviser, who called the advertisement unfair and defended Bain as doing “superbly well, acting within the rules, acting very responsibly,” a potential warning to the Obama camp that slamming Romney too hard for his past as a capitalist could backfire if voters choose to defend his work on free-market grounds.

The ads come at an especially important juncture in the general election contest between Obama and Romney. A CBS/New York Times poll released this week shows nearly one-third of voters hold no opinion of Mitt Romney, so both campaigns are trying to write a narrative about the candidate. Democrats want to paint him as an out-of-touch millionaire who made his fortune at the expense of middle-class workers; Republicans are trying to cast him as a job-creator who has on-the-job experience helping grow the private sector economy.

Foster, now an executive director of the Blue Green Alliance, predicted that Romney’s Bain years will eventually turn middle-class voters off to his candidacy.

“I think it will be a tough change of leadership for people to swallow,” he said.

Devin Henry can be reached at dhenry@minnpost.com. Follow him on Twitter: @dhenry