Amid anecdotal evidence on both sides, the full record has largely escaped a close look, because so many transactions are involved. The Wall Street Journal, aiming for a comprehensive assessment, examined 77 businesses Bain invested in while Mr. Romney led the firm from its 1984 start until early 1999, to see how they fared during Bain’s involvement and shortly afterward.

Among the findings: 22% either filed for bankruptcy reorganization or closed their doors by the end of the eighth year after Bain first invested, sometimes with substantial job losses. An additional 8% ran into so much trouble that all of the money Bain invested was lost.

Another finding was that Bain produced stellar returns for its investors—yet the bulk of these came from just a small number of its investments. Ten deals produced more than 70% of the dollar gains.

Some of those companies, too, later ran into trouble. Of the 10 businesses on which Bain investors scored their biggest gains, four later landed in bankruptcy court.