O’Reilly impales himself on a contradiction: He says his book is “laudatory” about Reagan — and that it is being attacked by Reagan “guardians” and “loyalists.” How odd. Liberals, who have long recognized that to discredit conservatism they must devalue Reagan’s presidency, surely are delighted with O’Reilly’s assistance. The diaspora of Reagan administration alumni, and the conservative movement, now recognize O’Reilly as an opportunistic interloper.

He began his profitable paltering with America’s past with “Killing Lincoln.” Historians advising the National Park Service, which administers Ford’s Theatre, found a multitude of errors in the first, uncorrected version, in which, for example, O’Reilly repeatedly places Lincoln in the Oval Office, which was built in 1909. The Theatre bookstore still does not sell “Killing Lincoln.” The Theatre gift shop, a commercial rather than educational entity, does. Four “histories” later, O’Reilly remains slipshod.

In “The Great Gatsby,” F. Scott Fitzgerald writes of Tom and Daisy Buchanan, who “smashed up things” and then “retreated back into . . . their vast carelessness . . . and let other people clean up the mess they had made.” Tidying up after O’Reilly could be a full-time job but usually is not worth the trouble. When, however, O’Reilly’s vast carelessness pollutes history and debases the historian’s craft, the mess is, unlike O’Reilly, to be taken seriously.