The American Berlusconi? One, this isn’t Italy. Two, 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 like Trump. My old friend Andrew Sullivan worries, persuasively, 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 Trump says he is going to do if he wins.

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To begin with, he would appoint his favourite judge to the Supreme Court, thanks to the Republican Party’s foolish decision not to confirm President Barack Obama’s nominee, Merrick Garland. As Trump told CNBC, he would also prevent the chair of the Federal Reserve, Janet Yellen, from serving a second term. In the same interview, Trump said he would 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 as president, 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 US companies that have the temerity to employ people abroad, on Day 1 of a Trump presidency, they would face fines. Finally, 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 recent rally.

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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 Obama’s. So would his new Fed chair. More importantly, according to the constitution, it is not the president but Congress that has the power to regulate the debt, immigration, taxation and trade. The president’s principal power lies in his being commander in chief of the armed forces. 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 convention to prevent Trump’s nomination – made it clear how he feels. Asked if he would endorse Trump, 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 Trump to scale.

Not-very-positive things foreign leaders have said about Donald Trump

For this reason, Trump may have to focus on foreign policy from the get-go. 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 President Vladimir Putin looks forward to it.

“A very bright and talented man,” was how Putin put it. Trump reciprocated. “He’s running his country and at least he’s a leader,” he said in December. In his foreign policy speech on April 27, Trump made it clear that he hopes to do a “great deal” with Putin. The prospect of Donald and Vlad consummating their bromance next year freezes the blood.

Putin sees in Trump the ultimate solvent of Western unity. Trump’s contempt for Nato is of a piece with his admiration of Putin, not to mention his enthusiasm for Brexit. An added bonus for the Russian president is Trump’s anti-Chinese rhetoric. Trump’s foreign policy would simultaneously break up the transatlantic alliance and sour the Sino-American relationship. What more could Putin ask for?

As the history of Latin America shows, populists are authoritarian, corrupt and economically ruinous for precisely the people they support. A Trump presidency would have the potential to be all of these things – but for the US constitution. Thanks to the genius of the founding fathers, 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? Do Republicans seek to field a third-party candidate, as proposed by Senator Ben Sasse? Do they prepare to work in a Trump administration in the hope of mitigating the damage? Or do they just shun him, in the knowledge that he is bound be a one-term president?

I was about to recommend the third course. Then I realised I’d just underestimated him again.

Niall Ferguson is Laurence A Tisch professor of history at Harvard and a senior fellow of the Hoover Institution, Stanford