IF ANYTHING COULD trip up former Massachusetts governor Mitt Romney in his pursuit of the Republican presidential nomination, it might be the charge that he and the private-equity firm he co-founded, Bain Capital, got rich dismantling companies and destroying jobs. This line of attack hits Mr. Romney at his strong point — his private-sector experience.

And so Texas Gov. Rick Perry blames Bain for layoffs in South Carolina, while a super PAC supportive of former House speaker Newt Gingrich is about to air a 27-minute film making similar claims. In his defense, Mr. Romney boasts that Bain helped create 100,000 jobs, adding that attacks on his Bain record amount to attacks on free enterprise.

Who’s right? Private-equity firms recruit investors — individuals, university endowments or pension funds — and use their money to start, restructure or expand businesses whose shares are not publicly traded. Once profits start flowing, the private-equity partners cash out, often by taking the company public. Of course, if the investment goes bust, the partners either lose money or make less than they hoped.

Risking your money on a business idea, and persuading others to join you, is Capitalism 101, a strange thing for conservatives like Mr. Perry and Mr. Gingrich to criticize — the latter with the “independent” help of a super PAC bankrolled by a casino magnate. Better that private parties take investment risks than government — as in the case of now-bankrupt Solyndra, or the hundreds of millions in state funds that Mr. Perry has steered to Texas high-tech firms.

Private equity strives to maximize returns to investors, and Bain certainly did that: to the tune of 88 percent per year between its 1984 founding and Mr. Romney’s retirement in 1999. Does it create jobs? Well, private-equity firms often shrink payroll to bring down costs. And some companies, laden with debt taken on to pay for restructuring, fail anyway. On the other hand, companies that survive restructuring can flourish and hire more people than they might have otherwise. Calculating net job creation depends on what-ifs that would be hard to specify, even if private-equity transactions were not, by their nature, less transparent than those involving publicly traded firms.

Mr. Romney and his partners at Bain astutely spotted and nurtured winners, and bet on their share of losers. Of 77 major deals that the firm did under Mr. Romney, 17 either filed for bankruptcy or closed their doors by the end of the eighth year after Bain first invested, according to the Wall Street Journal — a relatively high proportion that partly reflects Bain’s willingness to take on smaller, troubled firms. (Bain responded to the Journal that its figures included companies that went under even after Bain cashed out.) Among the success stories of Mr. Romney’s tenure are Staples and the Sports Authority. Those companies have thousands of employees, who might otherwise have worked at mom-and-pop competitors, or not at all. It’s hard to say how much credit belongs to Mr. Romney — mainly a rainmaker at Bain by his final years — and how much to other investors, or to workers and managers themselves.

A Bain-backed Kansas City steel mill, GS Technologies, went belly-up in 2001, at a cost of 750 jobs; but Steel Dynamics of Fort Wayne, in which Bain invested after more risk-averse financiers balked, is still going strong.

As a means of corporate finance, private equity is hardly evil per se. Probably it is one feature of U.S. capitalism that makes our system more flexible and capable of “creative destruction” than Europe’s.

Yet there are costs to putting firms through restructuring, so government policy should not unduly favor private equity. Current tax policy does just that. Private equity managers’ winnings count as “carried interest,” taxed at a much lower rate than ordinary income. Now there’s an issue.