It says something of how badly Donald Trump, the US Republican hopeful, has erred that journalists initially treated his statement calling for Muslims to be barred from entering the US as a hoax. Even he would not go that far, they said. Alas, they were wrong. He has, and he will continue to do so.

It has long been clear there are no limits to Mr Trump’s invective. The real-estate mogul has dealt in misogyny, racism, belittling of disabled people and schoolyard bully tactics. There is no reason to suppose his latest — and most egregious — outrage will be his worst. Although Mr Trump is exploiting the deep disaffection that many Americans feel towards Washington, his demagoguery has reached the point where it is safe to say that his politics are un-American. In the first instance, it will be up to his rivals within the Republican party to repudiate the warped message he is peddling. Their party’s future, and the character of US democracy, may depend on it.

On many levels, Mr Trump’s proposal sets off alarm bells. On moral grounds, closing US borders to an entire category of people is repugnant. The implication that every Muslim is a potential Isis recruit is offensive and unconstitutional: America enshrines an individual’s primacy before the law and admits to no religious bar.

During much of the 20th century, there was a de facto ban on Catholic immigration and in the late 19th century a bar on Chinese. Both episodes were aberrations from US moral and legal values. Mr Trump has already crossed the line by proposing a database for Muslim Americans. He declares they should also be required to carry separate identity cards. Each measure would be struck down by the courts. It is vital his latest broadside be defeated in the court of public opinion.

Mr Trump’s success in the Republican race is already harming the US national interest. Successive administrations have stuck to the line that the US is in conflict with a perverted form of Islam initially championed by al-Qaeda and now, in more virulent form, by Isis. The two presidents differed on whether to label it a “war on terror” — George W Bush’s designation that was subsequently dropped by Barack Obama. But they have been united in seeking to reassure Muslims that the US was not at war with their religion.

Mr Trump’s views play straight into the hands of Isis ideology, which depicts the US as an enemy of Islam. His invective will make Mr Obama’s job that much more difficult. It will also broaden the pool of sympathisers to the Isis worldview.

The reaction from Mr Trump’s fellow Republicans to his latest outburst has been encouraging. Jeb Bush, the former frontrunner, described Mr Trump as “unhinged”. Dick Cheney, the former vice-president, said his proposed Muslim ban “goes against everything we believe in”. Paul Ryan, the Speaker of the House of Representatives, said it was “un-American”.

Yet they have not gone far enough. Mr Ryan, for example, added that he would still support Mr Trump were he to be the Republican nominee. Others, including Mr Bush, have made Trump-lite noises. Mr Bush has suggested admitting only Christian Syrian refugees into the US. And so on.

It is time for Republicans to make clear they will not support Mr Trump under any circumstances. Such a stand may risk splitting the party. It may even risk boosting his support among sections of the rank and file. But it is preferable to the alternative. Mr Trump’s platform is antithetical to American values. The time has come for Republican leaders to broadcast that loud and clear.

Letters in response to this editorial:

Trump’s comments seem to have found acceptance / Chico Khan-Gandapur

Toughness is so tiring / From R Vijayaraghavan

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