The Republican party has traditionally been the predictable party when it comes to nominating a presidential nominee. But for 2016 everything has been tossed on its head. Donald Trump has a double-digit lead over his closest rival, according to a poll published Wednesday, leaving establishment figures trailing. The former reality television star in whom many are investing so much hope is also setting the terms of debate. The understandable frustration of many has transmogrified into a mindless attachment to a political harlequin. Something has gone awry in the party.

To understand how, one needs to understand the peculiar political currents in today’s US. Anti-political anger has descended on many Republican voters. The party’s leading candidates — Mr Trump and the neurosurgeon Ben Carson, neither of whom has governing experience — are benefiting.

This anger is, in some respects, justified. Political institutions have long been unresponsive to the challenges many Americans face, including stagnant wages, rising tuition and health costs, a byzantine tax code, high debt levels and mediocre education. Trust in politicians has fallen to the lowest level in 50 years, according to the Pew Research Center. Hence the appeal of an outsider such as Mr Trump, especially among male blue-collar workers, many of whom have borne the brunt of globalisation.

Mr Trump has also tapped into something that resonates with many of these Republicans: illegal immigration. This has undermined the rule of law and depressed the economic prospects of some low-skilled workers. But he has addressed the issue in a typically Trumpian way. He opts for extreme positions and incendiary language. He has implied that large numbers of those coming from Mexico are rapists and drug dealers, and advocated ending birthright citizenship and forcibly deporting millions of undocumented immigrants. The immigration issue, in turn, may be a proxy for those Americans, many of them older and working class, who feel they have “lost” their country and are fearful of the future.

But Mr Trump is ill informed on crucial issues. Answering a question about the Trans-Pacific Partnership, he railed against China, which is not part of the deal. He confused Iran’s Quds force with the Kurds. In the wake of the Isis attacks in Paris, he says, he “absolutely” wants a database of Muslims in America. He is also perpetuating the libel that “thousands and thousands” of Arab-Americans in New Jersey cheered as the Twin Towers crumbled on 9/11.

He has a tendency to inhabit a fairy-tale world. Mr Trump claims he will force Mexico to pay for the wall he wants built along its border. He claims that a President Trump would defeat Isis “very quickly”. Meanwhile, he purveys conspiratorial-sounding theories on subjects ranging from President Barack Obama’s birth certificate to the risks of childhood vaccination.

And then there is Mr Trump’s boorish manner. He has implied that a news anchor’s tough questions of him could be attributed to menstruation. He has ridiculed Senator John McCain’s ordeal as a prisoner of war. He has likened Mr Carson to a child molester with pathological tendencies. And last week he mocked a reporter with a disability.

It would be nice to chalk up his success to temporary insanity — an episode of Trumpmania that will end on its own. But a figure like Mr Trump does not appear ex nihilo. He is the product of certain intellectual and political habits that have taken hold over the years: a lazy anti-government ideology, prizing emotivism over empiricism, and conflict in pursuit of lost causes. This is not conservatism; it is splenetic, embittered populism. These habits of thought are discrediting the party of Abraham Lincoln and Ronald Reagan. Now would be a good time to begin to break them.

The Republican field boasts accomplished candidates — senators and governors — with serious reform agendas for the 21st century. The first primary is not until February, so it is not too late for Republicans to rally to one of them. They better had, because if they nominate Mr Trump, America’s avatar of irrationality, it will do grave harm to the party.

The writer, a senior fellow at the Ethics and Public Policy Center in Washington, served in the last three Republican administrations.