Sen. Rob PortmanRob PortmanConquering Trump returns to conservative summit ­ObamaCare fix hinges on Medicaid clash in Senate A guide to the committees: Senate MORE (R-Ohio), oft-mentioned as a possible running mate for Mitt Romney, offered a full-throated defense of the presumptive GOP presidential nominee's time at Bain Capital hours after President Obama's campaign launched a new ad critical of the private-equity firm.

Portman said the commercial — in which a former steelworker laid off when Bain liquidated his company describes the firm as a "vampire" — was an affront to free-market capitalism.

"That is capitalism. There aren’t different kinds of capitalism," Portman said at a lunch hosted by Bloomberg View, according to Buzzfeed. "There's the free market."

Portman went on to say that overall, Romney was responsible for positive job growth through some of Bain's more successful investments, and that the health of the American steel industry had little to do with his investments.

"They were trying to make a profit. Sometimes it worked sometimes it didn’t," Portman said. "Net, it did work though —he did create a lot of jobs."

Portman was also pressed about his potential addition to the Republican ticket, but said he wouldn't answer questions on whether he was being vetted by the Romney team.

"I think I'm better suited to stay where I am in the Senate," he said. "The folks in Ohio expect me to stick around and do my job."

But later, Portman did joke about the job — and managed a swipe against Senate candidate Elizabeth WarrenElizabeth WarrenSanders and Schumer are right: Ellison for DNC chair Dean: Schumer's endorsement 'kiss of death' for Ellison How the candidates for DNC chair stack up ahead of Saturday's vote MORE, whose Massachusetts race has been engulfed by questions over whether she falsely claimed Native American ancestry. Responding to a report Monday that a Romney aide described him — favorably — as a "boring white guy," the senator played off the Warren controversy.

"I am white — my Native American background notwithstanding," Portman quipped.

"I hope that I’m viewed as someone who gets things done, focuses on results," he continued. "That’s my goal in life, not to be exciting or boring, just to get the job done."

Portman also seemed to get a dig in at Vice President Biden, who raised eyebrows earlier this month when he said on "Meet the Press" that he was "comfortable" with gay marriage. That created a media firestorm that eventually resulted in Obama signaling his support for same-sex marriage and prompting Biden to apologize to the president for moving ahead of the White House on the issue.

"Vice Presidents have to go to funerals in Mozambique, they have to apologize for what they say," Portman said, downplaying the job.