He has been called a phoney, a fraud and a threat to democracy - and that is just by members of his own party. Other critics have compared Donald Trump to Hitler and Mussolini. I have shared in the widespread horror at Mr Trump's rise. But at the same time, a small voice in the back of my head has sometimes asked: "Is he really that bad? Might all this hysteria be a bit overdone?"
The death of fomer first lady Nancy Reagan over the weekend served as a reminder of the horror and condemnation that once greeted the rise of her husband Ronald Reagan. Like Mr Trump, Mr Reagan was labelled a fascist, accused of race baiting and derided as a moron. As a student when Mr Reagan took office, I remember the widespread predictions that he would provoke a world war. And yet he is now securely in the pantheon of "great presidents". Might Mr Trump make the same journey from odium to acceptance?
It is certainly possible to construct a case in Mr Trump's defence. If you move beyond the vulgarity of his style and examine his proposed policies, it is clear that on many domestic and foreign policy issues, he is more moderate than several of his "establishment" rivals for the Republican nomination. Mr Trump has argued for closing tax loopholes for high finance and has said the state should guarantee healthcare coverage for all Americans. He has also defended Planned Parenthood, a family planning group regularly savaged by the evangelical right.
When it comes to foreign policy, his rhetoric is aggressive and nationalistic. But his positions on specific issues suggest an Obama-like caution about US military intervention and democracy promotion overseas. Mr Trump's rivals are running attack ads against him for having the temerity to suggest, correctly, that it might not be a good idea to scrap the Iran nuclear deal. Also, in the last television debate, it was not Mr Trump, but the "mainstream" Republicans John Kasich and Marco Rubio who advocated the reckless option of sending American troops into Libya.
The most controversial Trump positions are his call for "the wall" along the Mexican frontier, and a temporary ban on Muslims entering the United States. Both policies have been deliberately phrased in provocative terms. But a demand for the enforcement of US border controls and for zero tolerance of illegal immigration is ultimately an argument for the enforcement of existing American law, which should not be an intrinsically outrageous idea.
The appeal for a ban on Muslims entering America was also immediately followed by the phrase, "until we've figured out what the hell is going on" - which is sufficiently vague to allow plenty of room to renege.
A large part of the horror at the Trump phenomenon is that he is appealing to "nasty" instincts among the American electorate. But it is the job of politicians to identify the concerns of voters, and to frame a response. There is a finer line than is commonly acknowledged between populism (which all "right-thinking" people abhor) and democracy (which we all approve of). Mr Trump has certainly revealed an acute and unorthodox political intelligence that has repeatedly left his rivals and the media flat-footed.
So that is the case for Mr Trump's defence. Do I buy it? Not really, I am afraid. The Trump phenomenon is as much about a political style, as about policies - and Mr Trump's style is abhorrent. His coarseness is a world away from the courtly good humour of Mr Reagan. Mr Trump has repeatedly used the most inflammatory language he can find, linking Mexican immigration to rape and endorsing the use of torture against terrorist suspects. It is not an accident that he has been embraced by leaders of the white supremacist movement, such as Mr David Duke.
Mr Mitt Romney, the Republicans' candidate in 2012, summed up Mr Trump's personal style well, when he decried "the bullying, the greed, the showing off, the misogyny, the absurd third-grade theatrics". It would send an awful message to society if those qualities were rewarded with the most powerful political office in the world. Mr Trump's vanity also makes him absurdly touchy, which is why he found it impossible, in the last debate, not to respond to a slur about the size of his penis. It is frightening to think that a man with this little self-control could be in charge of the world's largest nuclear arsenal.
Finally, Mr Trump's approach to the world goes well beyond an appropriate caution about American intervention overseas. He has also embraced trade protectionism and is highly sceptical about America's alliance commitments overseas. Those policies, if energetically pursued, would represent a full-scale retreat from US global leadership - which would probably make the world a much more dangerous place.
Of course, one of the fascinations of the Trump phenomenon is that it is very hard to know how much of what he says is for real and how much is posturing for the voters and the cameras. Mr Trump's record as a businessman and some of his writing suggest that he is, above all, a pragmatic deal maker who likes to make radical demands as an opening position, before finding a compromise.
It is certainly possible that a President Trump would surprise his critics and govern responsibly. But I sincerely hope that I never get to find out.
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