Donald Trump and the hidden history of the GOP.

• Author: Tanenhaus, Sam

Even before the early caucuses and primaries tested the actual strength of his robust poll numbers, Donald Trump had profoundly disrupted the Republican Party. But how, exactly?

The populist aspects of Trump's appeal look familiar--an aroused "radical center" alienated from "politics as usual"; economic anxiety; demographic changes that vex the Republican base. Yet Trump isn't the only Republican making this case. And his history and style differ from the normal insurgent's in our highly polarized moment.

He doesn't recite hot-button talking points (like Ted Cruz). He's not a defense hawk (like Marco Rubio), or Club for Growth supply-sider (like Jeb Bush). Nor does Trump seem a "movement" type. His praise of the Tea Party is devoid of sympathy for its hobbyhorse Constitutionalism: the calls to abolish the IRS and the Federal Reserve and repeal the Seventeenth Amendment, the fixation on states' rights.

Only Trump's xenophobia--his "birther" speculations about Obama, his denunciations of immigrants and Muslims--situates him on the right. Otherwise, he seems beyond ideology, or indifferent to it. He liked Planned Parenthood (until he didn't), and has said he's a fan of the single-payer health care systems in Canada and Scotland. And he seems ill-versed in conservative doctrine.

"He basically never says 'freedom' or 'liberty,'" the editors of National Review noted in October. "He talks only sparingly about the federal debt. He has, in short, ignored central and long-standing conservative tenets."

All this has made Trump not merely an outsider but an outlier, an unconservative enemy within the conservative gate. This is bad news for the GOP, Charles Krauthammer and Karl Rove warn, and could wreck the idea of "a conservative party as a constant presence in U.S. politics," says George Will. Another conservative, Ross Douthat, hears fascist overtones in Trump, complete with "popular elitism" or herrenvolk democracy.

The threat begins not in what Trump espouses. Much of it is familiar in our politics. It begins in his presentation: the bricolage of emotions and prejudices; the stream-of-consciousness orations; the gleeful tangling with hostile journalists and protesters; aggressive candor.

Trump may be sui generis, but he doesn't come from nowhere. The insurgency he's now leading is rooted in a long, but often neglected or submerged, strain of protest on the right. Its targets are "elites," who much of the time are themselves conservatives.

In 1954, the political writer Will Herberg, an ex-Communist moving steadily rightward, coined the term "government by rabblerousing." It was, he theorized, an...