I am not going to underestimate him again. In January, in a moment of weakness, I believed the assurance of a supposed expert that Mr Donald Trump's campaign to be the Republican presidential nominee would fizzle out when "real voting in real primaries" began.
I should have stuck to my longstanding view that, after big economic shocks, populism surges. Today Mr Trump is the presumptive nominee and the entire American political commentariat looks as useless as the economics profession did in 2008. The "econ" models failed to predict the financial crisis. The "pol sci" models failed to predict the political backlash. Note to self: Stick to history.
For this reason I am ignoring everyone who now says: "Oh, it'll be OK." In the past week, I have heard three bogus reasons to keep calm. He's not going to win the election. He'll just be an American Berlusconi, more bling and bunga-bunga than the nemesis of the republic. And, hey, he just tweeted a photo of himself eating a taco salad to celebrate the Cinco de Mayo fiesta. He loves Hispanics, really!
No, no and no again.
First, Mr Trump can beat the expected Democratic nominee Hillary Clinton. Forget the bookies who give him a one in four chance of being US president. Even if she does not end up being indicted for storing top-secret e-mail messages on her private server - a serious crime under the Espionage Act - Mrs Clinton personifies the political establishment that the public loathes and Mr Trump is running against.
And precisely because he is not a professional conservative, but a liberal on a number of social issues, Mr Trump is far more likely to lure white working-class voters away from Mrs Clinton than Mr Ted Cruz would have been.
The American Berlusconi? First, this isn't Italy. Second, Mr Trump can't afford to forget his election pledges and focus on partying. Remember, the first question a newly elected president asks himself is always: How do I get re-elected four years from now? To quote his adviser Roger Stone: "Having gone out a thousand times to say, 'I'm going to build a wall', he has to build a wall. He has said he would scrap trade deals; his voters will demand he scrap trade deals. He knows that."
The only half-decent argument for keeping calm is that the US Constitution was purpose-built to constrain a man such as Mr Trump. My old friend Andrew Sullivan worries that it is a less impregnable bulwark against tyranny than we assume. But to see why the separation of powers still matters, just consider what Mr Trump says he is going to do if he wins.
To begin with, he would appoint his favourite judge to the Supreme Court, which has one vacant position, thanks to the Republican Party's foolish decision not to confirm President Barack Obama's nominee, Mr Merrick Garland. As Mr Trump told CNBC last week, he would prevent Federal Reserve chairman Janet Yellen from serving a second term. He would also force foreign holders of US debt to accept cuts in the value of their bonds to reduce the federal debt.
By the end of his first 100 days in office, Mr Trump assured The New York Times, his wall along the Mexican border would be designed and his blanket ban on Muslim immigration would be in place.
As for those American companies that have the temerity to move operations overseas to save money, on day one of a Trump presidency they would face punitive fines. Finally, Mr Trump would impose an across-the-board tariff on Chinese imports. "We can't continue to allow China to rape our country," he declared at a rally last weekend.
Now for the good news.
He can do almost none of this if Congress opposes him. His Supreme Court nominee would also have to be confirmed, just like Mr Obama's. So would his new Fed chairman. More importantly, according to the Constitution (Article 1, Sections 7 and 8), it is not the president but Congress that has the power to regulate debt, immigration, taxation and trade.
The president's principal power lies in being commander-in-chief of the military. Even his right to make treaties is conditional on "the advice and consent" of the Senate.
In short, "the Donald's" anti-globalisation programme depends on his being able to muster majorities in Congress.
How easy is that going to be? Last week, the Speaker of the House - who some had vainly hoped could be drafted in at the Republican convention to prevent Mr Trump's nomination - made it clear how he felt. Asked if he would endorse Mr Trump, Mr Paul Ryan replied: "I'm just not ready to do that at this point. I'm not there right now."
Other Republican lawmakers are even more hostile. If, as seems probable, the Democrats win back control of the Senate in November, Capitol Hill will be more like a forbidding mountain for Mr Trump to scale. Like his predecessor, he would need to rely on executive orders to get around it.
For this reason, he may prefer to focus on foreign policy from the start. And here's where my worries really begin. For if there is one sure sign that a Trump presidency would be a disaster, it is the eagerness with which Mr Vladimir Putin looks forward to it.
"A very bright and talented man," is how the Russian President described him last year. Mr Trump reciprocated. "I think that I would probably get along with him very well," he said in October. "He's running his country and at least he's a leader," he added in December. "You know, unlike what we have in this country."
In his foreign policy speech on April 27, Mr Trump made it clear that he hoped to do a "great deal" with Mr Putin. The prospect of Donald and Vlad consummating their bromance in Moscow next year freezes the blood.
Mr Putin sees more in Mr Trump than just a kindred macho spirit. He sees the ultimate solvent of Western unity. Mr Trump's thinly veiled contempt for Nato is of a piece with his admiration of Mr Putin, not to mention his enthusiasm for Brexit. An added bonus for Mr Putin is Mr Trump's anti-Chinese rhetoric. The Donald's foreign policy would simultaneously break up the transatlantic alliance and sour the Sino-American relationship. What more could Mr Putin ask for (apart from a few more chunks of Ukraine and Syria)?
With few exceptions, as the history of Latin America shows, populists are authoritarian, corrupt and economically ruinous for precisely the people who support them. A Trump presidency would have the potential to be all these things - but for the US Constitution, which the president is sworn to uphold. Thanks to the genius of the founding fathers, Mr Trump is left only with foreign policy. But that is quite enough for him to be a global wrecking ball.
So how can he be stopped? Everything has failed so far: Jeb Bush, Marco Rubio, Ted Cruz. So "Republicans for Hillary" will probably fail too. That leaves sane Republicans to ponder even less palatable options.
Do they seek to field a third-party candidate, as proposed by Senator Ben Sasse, running the risk of helping Mr Trump win? Do they prepare to work with (or in) a Trump administration in the hope of mitigating the damage? Or do they shun him, knowing he is bound to be a one-term president?
I was about to recommend the third course. Then I realised I had just underestimated him again.
THE SUNDAY TIMES, LONDON
- Niall Ferguson is Laurence A. Tisch professor of history at Harvard and a senior fellow of the Hoover Institution, Stanford.
