Donald Trump is a force in the Republican presidential race. Two recent polls show him running second to Jeb Bush nationally. He’s also polling second in Iowa and New Hampshire. And his pronouncements are commanding attention, especially on Fox News, where he’s a popular presence.

Some of this is attributable to the fact that Mr. Trump is a genius at drawing the spotlight to himself. He thrives on social media. Democrats and the press are only too happy to highlight Mr. Trump’s stream of invective and outrageous utterances, including his claim that Mexico is purposely sending us drug dealers, rapists and carriers of infectious diseases.

To their credit, several Republican presidential candidates, including Rick Perry, Marco Rubio and Mr. Bush, have criticized Mr. Trump. Yet it’s also true that Mr. Trump has garnered respect from some of his fellow Republicans. According to Ted Cruz, Mr. Trump is “bold,” “brash” and “speaks the truth.” Rick Santorum likes it that he’s focused on “a very important issue for American workers and particularly, legal immigrants in this country.” An editorial in The Weekly Standard says Republicans can benefit from “a little touch of Trump” in rhetoric, attitude and bearing. According to Rush Limbaugh, Mr. Trump’s statements will “resonate” with many Americans.

They may. But they shouldn’t.

For starters, Mr. Trump, though he claims to be a conservative, is nothing of the sort. He’s barely even a Republican. For most of the last decade, he was a registered Democrat. It wasn’t that long ago that most of his political contributions went to Democrats, including Senators Harry Reid, Chuck Schumer and Hillary Clinton. Before he discovered his Republican roots, Mr. Trump favored a single-payer health care system and proposed a large “net worth tax” on wealthy individuals. He once declared himself “strongly pro-choice” and favored drug legalization. He is a vehement protectionist. Earlier this year he even accused Republicans running for president of “attacking” Social Security, Medicare and Medicaid. Why would conservatives find him the least bit appealing?

Mr. Trump has no coherent governing philosophy. All he has is an attitude, and a crude one at that. As his announcement speech and subsequent statements have made clear, his command of the issues is superficial, his presentation often rambling and demagogic.

At the heart of Donald Trump’s candidacy is folie de grandeur. Mr. Trump will build a fence on the southern border — and get Mexico to pay for it. He’s got a “foolproof” plan to defeat the Islamic State “very quickly,” but when asked what it is, he told Fox’s Greta Van Susteren, “I’m not gonna tell you what it is tonight.” He’ll have a “great relationship” with Vladimir V. Putin while also keeping Iran from getting a nuclear weapon. His policy views on China consist mostly of bluster (“Oh, would China be in trouble. The poor Chinese.”). Mr. Trump is eager to tell us that “there’s nobody bigger and better at the military than I am.” He also gets things done “better than anybody.” And he will be “the greatest jobs president that God ever created.”

Mr. Trump thinks he’ll snap his fingers and everyone from his political opponents to foreign nations will bend to his will. He is a man operating on his own fantasy island.

There’s also the matter of Mr. Trump’s erratic and conspiratorial temperament. He believes “massive vaccinations” cause autism (“the doctors lied”). He was the most prominent person in America pushing the theory that President Obama — a “psycho!” and “maybe” a Muslim — might not be a natural-born American citizen. And he delights in making malicious statements. He’s absolutely sure, for example, that Mexicans crossing our southern border are drug dealers and rapists, but he can only “assume” there are “some good people” crossing the border as well. (Mr. Trump would later say that “many fabulous people come in from Mexico” and “I get along great with Mexico. I get along great.” Of course he does.)

As for political discourse, Mr. Trump is insulting and witless. His critics are “losers,” “jerks” and “dummies.” These kinds of taunts are usually found in schoolyards rather than presidential campaigns. But a presidential campaign is right where he is.

There’s not much that can be done about that. If conservatives rally to defend Mr. Trump on the grounds that he’s “refreshing” and has “passion,” that he’s “anti-establishment” and irritates liberals, they will do considerable damage to their movement and to the Republican Party. Mr. Trump is a pernicious figure on the American political landscape. He can’t be wished away. Which means the people who have to confront and expose him are conservatives. We’re the ones who have the most to lose from a successful Trump candidacy.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a contributing opinion writer.