You’ve tried shredding his record, mocking his ideology, assailing his truthfulness. It’s come to not very much. So if you’re Newt Gingrich or Rick Santorum or Jon Huntsman or Ron Paul, how, exactly, do you stop Mitt Romney?

Political Times Matt Bai’s analysis and commentary.

The answer is: You probably don’t, or at least not in any mortal way. But in this thing about Bain Capital and the factories it closed, Mr. Romney’s rivals might just have found something he needs to worry about, now and in November.

Like it or not, and most of us don’t, there is a real craft to negative campaigning. For an allegation to do real damage, it has to confirm some narrative about a candidate that voters already fear. And it’s always more effective to undermine the strength of a candidate than it is to underscore a known weakness.

Liberals trying to un-elect George W. Bush in 2004 published entire indictments enumerating his crimes against the English language. None of it amounted to much, because most voters had long since decided that Mr. Bush wasn’t fit for Mensa, and they didn’t especially care.

But conservative attacks on John Kerry’s war record were devastating, precisely because they went directly to the core of his argument. Not only did the “swift-boating” of Mr. Kerry cast doubt on his heroism in Vietnam, but it also added to lingering doubts about his constancy, suggesting that he would change his story when the circumstances demanded it.

To this point, the attacks lobbed at Mr. Romney have been disparate and not terribly persuasive. Like a man hurling whatever stone he can find lying around, Mr. Gingrich has assailed Mr. Romney for lying about negative ads, for changing positions more often than a yogi, for being too moderate. The problem with all of these lines of attack is that they don’t tell anyone anything new.

No Republican who’s voting for Mr. Romney is doing so because they think he’s a granite-spined social conservative who never wavers in his convictions. Romney voters like him because they think he’s the guy who knows how to create jobs, and because supposedly he can win over enough independent voters to unseat President Obama.

(The ads that Mr. Romney’s allies ran against Mr. Gingrich in Iowa, on the other hand, proved eviscerating because they annihilated his confrontational persona. The video of the former speaker sharing that couch with Nancy Pelosi couldn’t have been more disturbing to conservatives had it been the grainy tape from some convenience store, showing Mr. Gingrich with a hoodie and pistol.)

This Bain stuff, however, the 27-minute video and the accompanying ad campaign in South Carolina, is something else. The basic gist, in case you missed it, is that Mr. Romney’s private equity firm routinely made money by buying struggling companies and shuttering their plants — including one in Gaffney, S.C., where more than a hundred steel workers were reportedly laid off.

The attack could prove sticky, for three reasons. First, it takes Mr. Romney’s central rationale as a candidate and turns it into a bludgeoning tool. Mr. Romney, after all, is the guy who knows all about turnarounds, which is why we’re supposed to hire him as our c.e.o. Every day he is forced to defend his business record, as opposed to his stance on abortion or gun control, is a bad day for Mitt Romney.

Second, it casts doubt on Mr. Romney’s aura of electability. Most independent voters may not be ready to take up signs and occupy the local park, but neither are they feeling especially warmly toward Wall Street speculators and bonus-gobbling chief executives at the moment, and the Bain narrative casts Mr. Romney in exactly this light.

And third, the Bain line of attack, more than anything else brandished against Mr. Romney to this point, might bring to the surface an instinctive concern that he’s emotively challenged. I heard some version of this a lot when I visited conservative activists and operatives in South Carolina a few weeks ago — that Mr. Romney seemed plastic and programmed, an impression that could only be exacerbated by the idea that he was laying people off and sleeping just fine.

For all these reasons, you can potentially see the darker side of private equity becoming for Mr. Romney, either in coming primaries or in the fall election, what the morally ambiguous side of soldiering became for Mr. Kerry in 2004. Namely: a means of discrediting his central argument for the White House, and an issue that could all too easily, and unfairly, come to define his essential character.

If Mr. Romney’s rivals stick around and press this case effectively, they may yet keep this primary season alive for several more weeks. If Mr. Romney can’t find a way to quickly blunt the attack, he may be answering for it long after that.