Always quick to get to the point, social media came up with an instant distillation of the global response to the improbable, unsettling election of Donald Trump as president of the United States: #RIPAmerica.

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That hashtag declared that we had witnessed something more than a simple change of government. (Such a slogan would not have circulated had Mitt Romney beaten Barack Obama in 2012.) Instead, it implied that Trump had not merely taken over the running of the United States for four years, but that his presidency represented a much darker threat – that it would, in fact, destroy the country.

Of course, Twitter is prone to the hyperbolic and hysterical. But more sober commentators were raising a similar prospect, albeit in not quite such stark terms. David Remnick, the editor of the New Yorker, spoke for many when he wrote: “The election of Donald Trump to the presidency is nothing less than a tragedy for the American republic, a tragedy for the constitution, and a … sickening event in the history of the United States and liberal democracy … It is impossible to react to this moment with anything less than revulsion and profound anxiety.” For the millions who shared that sentiment, dawn on Wednesday felt like mourning in America.

What they fear is that Trump will make good on all the promises – and threats – he made during these last 18 months. What if he goes ahead and deports 11.3 million undocumented migrants? What if he really does ban all Muslims entering the country? What if he tries to use the powers of the state to go after media organisations that have criticised him – making life difficult for the businesses that own inquisitive newspapers such as the Washington Post, for example – as he has said he will? What if he overturns abortion rights, even imposing “some form of punishment” on a woman who terminates a pregnancy, as he once suggested? And what if he really does build that wall?

There are plenty who believe that if Trump went ahead and actually implemented his programme, he would create a different country: closed, xenophobic and at odds with some of the founding principles – religious equality or freedom of speech – that have defined the United States since its founding. The country would still exist – but it would no longer be America.

Naturally, there is a ready chorus of self-styled realists and grownups, quick to say that such talk is excessive. Those voices offer assorted forms of reassurance. First, they make the perennial claim that every successful politician – whatever the bluster and rhetoric of the campaign – always moderates once in office. This view holds it to be all but a law of nature that the radicalism of the candidate is always tempered by the reality of governing. Trump may have talked up a storm on his way to the White House, they say, but once behind the desk of the Oval Office, pragmatic, practical considerations will surely constrain him.

What’s more, runs the argument, there are formal mechanisms in place to do just that. The United States, after all, is ruled by a constitution that insists on the separation of powers, so that the executive – the president – can never go too far, always held in check by both the independent judiciary, in the form of the supreme court, and the legislature, Congress. These venerated institutions will surely prevent Trump from doing anything too crazy. Less august, perhaps, but also to be taken into account is the federal bureaucracy: a vast civil service of non-partisan technocrats, who will ensure that any Trump proposal is softened and smoothed into shape, sanding off its harsher, more wild edges. And there is always the military: they surely won’t let Trump get out of hand.

The trouble is, none of those reassurances stacks up. Start with the notion that once he has sworn the oath of office, Trump is bound to tone it down, dropping his most incendiary plans. The trouble with that idea is that party bigwigs were saying the same thing throughout the campaign, earnestly hoping that Trump would change once he became the frontrunner, that he would change once he won the party nomination, that he would change once he started the campaign proper in September. Normal candidates do indeed do that. But with Trump, it never happened.

Facebook Twitter Pinterest Jaden Rams cheers as Donald Trump speaks at a rally in Grand Junction, Colorado. Photograph: Damon Winter/New York Times/Redux/Eyevine

Instead, he kept sounding off, insulting bereaved military parents one minute, a former Miss Universe the next. The only time he agreed to take direction was in the very final week, when he finally stuck to a message and read scripted lines from a teleprompter. But that was just for a few days, when the big shiny prize was in sight. It is a slim foundation on which to build the hope that a 70-year-old man with a proven lack of impulse control – “I just grab them by the pussy” – will miraculously transform himself the minute he walks through the door of 1600 Pennsylvania Avenue.

But if the restraint won’t come from within, what about from without? Won’t there be institutions to hold Trump in check? The trouble is, Republicans did not only win the presidency, they also won both houses of Congress. That gives Trump enormous power, a strength that has eluded most of his predecessors. (George W Bush governed with a Republican Congress for a short spell. Before him, the last Republican to enjoy such untrammelled might was Herbert Hoover in 1928.)

The result is that Trump will face no congressional constraint. They will do his bidding. Some have suggested that, while their party affiliation might be the same, Trump will nevertheless encounter resistance on Capitol Hill because so many congressional Republicans are hostile to him. They cite the House speaker, Paul Ryan, who publicly clashed with Trump several times on the campaign – once accusing him of making a “textbook example of a racist comment” – and who endorsed Trump only through gritted teeth.

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But Ryan will not dare oppose Trump now. Trump is the one with the national mandate, the winner of a much larger electoral college victory than anyone ever thought possible. Trump won states that had eluded the Republicans for decades: he even took Pennsylvania, which had been in Democratic hands since 1988. That gives him huge clout inside the Republican party. He succeeded in doing what they had long failed to do, expanding the map and winning over white working-class voters who had previously spurned them.

And so Republicans in Congress will be the junior partners in this new relationship, forced to bow to the man whose electoral appeal is so much stronger than theirs. He won’t be their creature; they will be his. If representatives were to dare to step out of line, Trump will ask them who the voters in their districts – who will almost certainly have voted for him – would back. The answer will always be him.

That pressure won’t be applied only to Republicans. Witness this exchange between Newt Gingrich, rumoured to be a possible secretary of state in a Trump administration, and journalist Evan Osnos. Osnos had asked about the proposed border wall with Mexico, wondering if a Trump administration would really be able to get Congress to pay for it (given that no one believes Mexico ever would, despite Trump’s hot air, pay for it itself). Gingrich replied that he would use the pressure of re-election to bring doubters around. “Remember how many Democrats are up for election in the Senate in 2018,” Gingrich said, adding that the answer was 25. “Do you really want to go home as the guy who stopped the fence? Then, by all means, but we’ll build it in ’19.”

So if Congress promises to be supine, what of the supreme court, surely the ultimate restraint? That body has a vacancy, ready to be filled by the next president. Trump can nominate a pliant judge and the Senate will be unlikely to resist. That would instantly give Trump a 5-4 conservative majority on the highest court. And with several other supreme court justices in their late 70s or 80s, other vacancies could appear soon, too. Trump will have the chance to reshape the court in his image. (He can do the same for the lower courts, too, rapidly filling up the federal bench with Trump-friendly lawyers.)

Facebook Twitter Pinterest A man holds up an image of Barack Obama with the word ‘tyrant’ above itduring a Trump rally in Colorado Springs. Photograph: Damon Winter/New York Times/Redux/Eyevine

And remember, once on the court, a judge can stay for life. So Trump’s impact will be felt for a generation. He could have the numbers to overturn Roe v Wade, the ruling that gives women abortion rights; to further entrench so-called gun rights, reducing what few restrictions currently exist (he has proposed that schoolteachers be armed); or even to reinterpret the first amendment to allow for his proposed tightening of the libel laws: Trump wants the right to pursue journalists who have criticised him, so that “we can sue them and win lots of money”.

As for the federal bureaucracy, the US system is not like the one in Britain. A new president does not have to confine himself to appointing a handful of special advisers, fitting them into an existing, politically neutral machine. He makes a staggering 4,000 appointments, staffing the entire top layer of government – all owing their jobs and total loyalty to him. So, no, there will be no civil service to stop Donald Trump. They will be Trump’s servants. (It’s said that the military will behave in the same way: they will not want to be seen resisting democratically elected, civilian power.)

In other words, Trump will have enormous power from inauguration day on 20 January 2017. He can build his border wall if he wants to – and, so memorable was the promise, the political penalty for not doing so will be enormous. But he can do much, much more besides.

On day one, he can – at the stroke of a pen – undo much of the Obama legacy. Trump has had a team of aides draw up what they call the First Day Project, a series of executive orders signed by Obama that Trump could overturn with his signature before he has got his feet under the desk. Among the measures they have targeted for repeal are: the Paris agreement on climate change, the Syrian refugee programme and the rules demanding checks on the background of someone seeking to buy a gun. By the end of that first day, America would already be a changed country.

Once work starts on the border wall, Trump’s most vociferous supporters will demand action on the president-elect’s other signature proposal: the deportation of those migrants. With Republican control of the House and Senate, he will have no excuses for inaction or delay. On this, too, there has been some advance work. One thinktank envisages a force of 90,000 “apprehension personnel”, equipped to make raids on farms, restaurants and building sites, rooting out illegal immigrants. As Osnos reported: “Thousands of chartered buses (54 seats on average) and planes (which can accommodate 135) would carry deportees to the border or to their home countries.” Others have talked of putting these unwanted people on trains and expelling them – a method that evokes the darkest chapter in European history.

In the meantime, Trump could work on his ban on Muslims entering the US. The constitution might strike that down as discrimination on grounds of religion, but it would not be too hard to refashion it. Indeed, Trump has spoken of banning arrivals from countries associated with “Islamic terrorism”, which may be an effective workaround. And all the while, Trump would be shredding the legacy of America’s first black president, abolishing Obamacare which gave millions health insurance.

But there is another promise Trump will want to implement. It came during the TV debates when Trump pledged that, if he won, he would put Hillary Clinton in jail over her use of a private email server as secretary of state. He can’t do that directly, but he could steer his Justice Department to pursue her, perhaps via a special prosecutor. Who would stop him doing that? His base will demand nothing less: their cries of “Lock her up!” are still ringing. (The only thing that might stay his hand is if he calculates that the threat of prosecution provides useful leverage, a sword to hang over the heads of the Clintons, to ensure they keep quiet and out of his way.)

So this is the country that Trump – unshackled and unbound – can forge, starting three months from now. Oblivious to the environment, hostile to migrants, snarling towards Muslims and seeking to roll back rights that took decades to secure. It would be a country that saw the constitution not as its greatest gift, but as an obstacle to overcome. It would see diversity not as a strength, but as a fact to be tolerated, at best. And anyone objecting to any of this would find the big institutions, all three branches of government, arrayed against them. RIP America? It certainly might feel that way.

For what we saw in the astonishingly ugly campaign fought by Donald Trump would be only an inkling of what is to follow. Or, as one senior Democrat put it to me: “The horror show has not even started. This is just the overture.”