The debt helps juice the firms’ investment returns—as with any investment, the less you put down, the higher your returns will be (assuming things don’t go bust). It also makes them dependent, to a great degree, on credit markets—when credit is loose, as it was for most of the aughts, it’s easy for private-equity firms to make their returns look good (at least in the short run). When credit gets tighter, as it did after 2007, it becomes much harder to do so. As one private-equity manager says in a recent paper by Ulf Strömberg, Tim Jenkinson, Per Axelson, and Michael Weisbach, “Things are really tough because the banks are only lending 4 times cash flow, when they used to lend 6 times cash flow. We can’t make our deals profitable anymore.”

Debt is also valuable to private-equity firms because the government subsidizes their borrowing—corporate interest payments on debt are tax-deductible, while dividend payments (which you can think of as payments on equity) are not. And this tax-subsidized borrowing has been a key part of their success over the years.

The University of Chicago’s Steve Kaplan, arguably the leading researcher in the field and an unabashed defender of private equity, recently wrote a piece extolling private-equity firms’ performance, and attributing their superior returns to their ability to better incentivize and supervise management and their ability to introduce operational improvements.

What Kaplan strangely fails to mention are the advantages created by all the debt these firms take on, even though, in a paper co-authored with Per Strömberg, Kaplan himself wrote that the debt subsidy alone for L.B.O.s in the nineteen-eighties may well have accounted for ten to twenty per cent of the value these deals created. Even if private-equity firms are, in some circumstances, able to do a better job of managing companies, a substantial portion of their superior returns is due simply to the fact that they exploit the U.S. tax code better than most public companies do.

There’s nothing inherently wrong with debt, of course. But if the boom and bust of the past decade taught us anything, it’s that too much private-sector borrowing makes the economy more volatile and unstable. So if the G.O.P. is serious about the perils of excessive borrowing, it could start by limiting the deductibility of corporate debt, which would force companies to rely more on equity financing and would discourage the kind of credit bubbles we’ve seen over the past fifteen years.

Don’t hold your breath waiting for Mitt Romney to do anything about this, though, since his fortune, as we saw from his tax returns this week, has been created by corporate borrowing. And, even as the G.O.P. continues to inveigh against the perils of deficits, the implicit message it’ll send by nominating Romney is quite different: Debt for me, but not for thee.