The most embarrassing moments to watch this political season have occurred as Mitt Romney has pretended to be an angry, fire-breathing true conservative. The evidence suggests that in his soul he’s a moderate pragmatist, but he has flip-flopped like a frantic fish in hopes of hiding his reasonableness.

Newt Gingrich, Romney’s main rival for the Republican presidential nomination, is denouncing Romney with one of the ugliest slurs in the Republican lexicon: a Massachusetts moderate. Other moderate Republicans are savaged as RINOs — Republicans in name only — as if they emerged from an ugly mutant strain.

Yet, in fact, as a new history book underscores, it is the Gingriches and Santorums who are the mutants. For most of its history, the Republican Party was dominated by those closer to Romney than to social conservatives like Rick Santorum, and it is only in the last generation that the party has lurched to the hard right.

The new book, “Rule and Ruin,” by Geoffrey Kabaservice, a former assistant history professor at Yale, notes that, to compete in the primaries, Romney has had to flee from his own political record and that of his father, George Romney, a former governor of Michigan who is a symbol of mainstream moderation.