The historian Howard Zinn won a dubious prize of sorts last year when his best-selling “People’s History of the United States” came in second in an informal online poll to determine the “least credible history book in print.”

But now some of Mr. Zinn’s strongest scholarly critics have rushed to his defense, following the revelation that former Gov. Mitch Daniels of Indiana had, while in office, sent e-mails to a state education official asking for assurance that Mr. Zinn’s “truly execrable, antifactual piece of disinformation” was “not in use” in Indiana classrooms....