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 past 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.

“Much of the current conservative movement is characterized by this sort of historical amnesia and symbolic parricide, which seeks to undo key aspects of the Republican legacy such as Reagan’s elimination of corporate tax loopholes, Nixon’s environmental and labor safety programs, and a variety of GOP achievements in civil rights, civil liberties and good government reforms,” Kabaservice writes. “In the long view of history, it is really today’s conservatives who are ‘Republicans in name only.’”

After all, the original Massachusetts moderates were legendary figures in Republican history, like Elihu Root and Henry Cabot Lodge. Theodore Roosevelt embraced progressivism as “the highest and wisest form of conservatism.” Few did more to promote racial integration, civil rights and individual freedoms than a Republican, Earl Warren, in his years as chief justice.

Dwight Eisenhower cautioned against excess military spending as “a theft from those who hunger and are not fed.” Richard Nixon proposed health care reform. Ronald Reagan endorsed the same tax rate for capital gains as for earned income.

Republican history is also populated with harder-line conservatives, like Sen. Robert Taft of Ohio, dubbed “Mr. Republican.” But he worked closely with Democrats, was willing to raise taxes and disapproved of anti-intellectual populism.

What happened?

That’s a long and gradual story beginning with Sen. Joe McCarthy’s success in galvanizing working-class suspicions of government elites and continues with an angry backlash at changing mores and liberalized abortion laws. Conservative Southern whites moved into the Republican Party. Newer media voices like Rush Limbaugh and Glenn Beck made extremism seem congenial — while making bipartisanship feel treacherous.

I grew up in Oregon at a time when the state was dominated by Republican progressives like Gov. Tom McCall, a passionate environmentalist, and Mark Hatfield, an opponent of the Vietnam War. At that time, political paranoiacs tended to vote Democratic for candidates like George Wallace; over time, they migrated to the state’s Republican Party — and swallowed it up.

Yet political parties are not suicidal. When they overreach, they (often) learn. The Democrats did that when they embraced a Southern centrist named Bill Clinton. And in Oregon over the past decade, Bladine notes, social wedge issues have lost their force, and moderate Republicans have re-emerged.

Could the same happen nationally? Sure, it seems impossible at the moment. But if Romney somehow manages to make the Republican Party safe for moderates again, that’ll be a triumph for his party — and for the country.