Though he’s repeatedly vowed to avoid negative campaigning, Newt Gingrich took a pretty sharp swipe at Mitt Romney today, using one of Romney’s biggest vulnerabilities against him.

On Fox News earlier, Romney went after Gingrich on the money he received from Freddie Mac. Soon after, campaign reporters asked the disgraced former House Speaker for his response.

For those who can’t watch videos online, Gingrich said:

“I would just say that if Gov. Romney would like to give back all of the money he’s earned from bankrupting companies and laying off employees over his years at Bain, that I would be glad to then listen to him. I’ll bet you $10, not $10,000, that he won’t take the offer.”

Ouch. I guess everyone really is taking off the gloves.

Keep in mind, Gingrich’s attack has the benefit of being true. Romney’s private-equity firm enjoyed playing with borrowed money, orchestrating leveraged buyouts, seeking taxpayer subsidies, flipping companies quickly for large profits, and in true vulture-capital style, laying off scores of American workers. Romney was apparently good at it, making money for investors “even when companies slid into bankruptcy.”

I’ve been wondering for a long while why this has been a non-issue in the Republican presidential race. Apparently, that’s changing.

Keep in mind, the potency of the Bain Capitol attack has been utilized by Romney critics in both parties. Ted Kennedy used the issue to great effect in 1994, and Mike Huckabee embraced it in 2007 (“I want to be a president who reminds you of the guy you work with, not the guy who laid you off.”) Gingrich is using it now, and if given the opportunity, President Obama will use it in 2012.

The line stings because it’s true.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.