Mitt Romney's political foes are stepping up attacks based on his time running investment firm Bain Capital, tagging him with making a fortune from the rougher side of American capitalism—even as Mr. Romney says his Bain tenure shows he knows how to build businesses.

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...