Sen. Mark Warner (D-Va.) weighed in on the debate over Bain Capital Tuesday, calling Mitt Romney’s former firm “a valid topic of debate” but also praising it as “a very successful business” that did “what they were supposed to do.”

Warner made millions in venture capital before being elected governor and senator, so it’s not surprising he would be asked his thoughts on the Obama campaign’s attacks on the company Romney, the former Massachusetts governor and presumptive Republican presidential nominee, used to run. The discussion has heated up in recent days in the wake of Newark Mayor Cory Booker’s (D) mixed answers on the subject.

Warner, a moderate who prizes bipartisan compromise, took a middle ground on MSNBC (beginning at the 5:45 mark below).

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MSNBC host Chuck Todd asked Warner whether he was “comfortable” with Obama’s “attacks on Bain Capital? Do you think it’s an attack on private equity?”

Warner said he was “proud” of his work in venture capital, adding: “One of the things I found as I made the transition to public service, there is a different skill set. Oftentimes if you’ve got investors, you’re looking for short-term profits. If you’re in public service, if you’re investing in pre-school, if you’re investing in infrastructure, you’ve got a much different time horizon. There is a different skill set, and I think it’s a valid topic of debate.”

Asked whether Bain “practiced good or bad ethics,” Warner said, “I think Bain Capital was a very successful business. I think they got a good return for their investors. That is what they were supposed to do.” Republicans quickly seized on the comment, blasting it out to their press list.

But Warner then elaborated on his point in a way that could undercut Romney’s campaign message.

“I think when you’re in public life, though, what you’ve got is a different time horizon,” Warner said. “The notion that everything in government is exactly the same way that it is in business, they’re different time horizons when you’ve got to invest for the long haul, when you’ve got to actually do the kind of early stage investing, whether in preschool, whether its in K-12, whether it’s in infrastructure that don’t pay back quarter to quarter.”