But Mr. Gingrich was himself on an advisory board for a major investment firm that had a similar business model, Forstmann Little, a pioneering private equity firm co-founded in 1978 by Theodore J. Forstmann that was, along with Mr. Romney’s Bain Capital and Henry R. Kravis‘s Kohlberg Kravis & Roberts, among the leading private equity firms during the 1980s and 1990s.

Forstmann Little earned billions of dollars in profits from its investments in companies including General Instrument and Gulfstream Aerospace. But the firm shut down most of its operations a decade ago after suffering losses from ill-timed bets on highflying telecommunications companies at the height of that industry’s bubble.

Mr .Gingrich’s involvement with the firm could complicate his attacks on Mr. Romney.

Still, to be fair, Mr. Forstman bristled at some of the more aggressive tactics of his rivals, and once described them as “barbarians at the gate.”