Remember that terrific scene at the end of the movie The Candidate, when one Bill McKay (Robert Redford) has just amazed himself and everyone else by winning a California Senate seat against an incumbent who was supposed to be invincible? “Marvin,” he asks his campaign manager, “What do we do now?”

You wonder whether the same question doesn’t apply to Donald Trump right now. The two protagonists of course are not exactly cut from the same cloth. The movie, made in 1972, features a Redford who is young, impossibly handsome and fetchingly idealistic – not qualities associated with the jowly, perennially pouting Trump, who at 70 will be the oldest president ever to take office. But one thing they have in common: they didn’t expect to win.

Trump does have one obvious answer to The Candidate question: he’s going to drain the swamp. After all he’s a complete outsider, whose experience is in business, not politics,who’s just defeated a supreme insider (who was also supposed to be invincible.) He has a mandate to place a bomb under the entrenched ruling establishment in Washington, so out of touch with, and indifferent to, the voters who elected him.

Technically, he’s a Republican. But in spirit Trump is an independent protest candidate, smart enough to realise the only way he could win was under the aegis of one of America’s two major parties. On his own, he’d probably never have done it, ending up like another businessman, Ross Perot, who won 19 per cent of the vote in 1992, but carried not a single state.

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If he is going to drain the swamp, the stage appears perfectly set. This is one of those rare moments when a president comes to power with his side controlling, or about to control, all three branches of government: the White House, Congress, and as soon as he has filled its outstanding vacancy, the Supreme Court as well.

As a successful businessman, he’s presumed to be efficient, with an unsentimental fixation on the bottom line. If it works, keep it, if it doesn’t, junk it. He owes no favours to his party, which largely rejected him, but which partly owes its continuing control of the House and Senate to, get this, Trump’s own coat-tails. Who’d have thought it?

Not so fast however. Government doesn’t work like business. The goal of business is to make money. Successful government is a far more complicated and inexact matter. It’s about advancing the common weal, sometimes by massive investments with no financial returns that no business would contemplate. It’s about improving relations with other countries, it’s about making as much of the citizenry as possible feel it has a stake in what’s being done.

Nor is it a question of barking out orders that will be instantly obeyed, as corporate leaders expect. In government, there are tiresome things to be dealt with like the supermajority of votes in the Senate (which the Republican’s don’t have) to pass any significant legislation. There are the courts, and the entire system of checks and balances contained in the Constitution, created by the Founding Fathers deliberately to prevent dangerous demagogues running riot.

Next are the entrenched bureaucracies of Washington. They tremble now, but you can bet they will find ways of resisting, of quietly obstructing ideas that threaten them. And do not forget the lobbyists, the most permanent governing class of all, hub of the city’s great money-go-round. Experts already predict the next administration, for all Trump’s fulminating against lobbyists, will in fact be a bonanza for them.

The real bottom line is that a US president’s power, domestically at least, is limited – much more so, for example, than a British prime minister with a Commons majority. Theodore Roosevelt was right when he pointed to the “bully pulpit” power of the office. The soon to be 45th president most certainly is a bully. But for the pulpit part to work, you need a clear majority of the country behind you. Which, right now at least, is manifestly not the case with Donald Trump.

And one other thing is required for a would-be iconoclast in the Oval Office, a laser-like focus on your strategic objectives. The evidence so far suggests that in Trump’s case, pretty much the opposite is true.

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By all accounts, he has an extremely limited attention span. He’s a “gut” politician who goes by his instincts, usually after the briefest of consideration. He never bones up on an issue; indeed, it is said he’s never finished a book. He’s desperately thin-skinned where his image is concerned, and easily distracted by a slight, real or imagined (see campaign 2016, passim.) He’s absurdly litigious, and bears grudges long past their sell-by date.

None of this is automatically disqualifying. George W Bush was a “gut” president, never one for lengthy and complicated briefs. But Bush, you always felt, had a broad strategy, love it or loathe it. Trump has shown little sense of strategy or long-term planning. Everything is seat-of-the-pants. Can you run a country by tweet? We shall see.

On the plus side, Trump vaunts the businessman’s ability to cut deals (though several of his – the casino projects that ended in bankruptcy, the purchase of the Trump Shuttle and the Plaza Hotel – were, to use a favourite Trump term, disasters). He’s also an unarguably brilliant communicator, who understands TV better than any politician. And he has a razor-keen political instinct that enabled him to see the boiling discontent of the heartlands that almost every pundit missed.

Yes, we owe Trump “an open mind and a chance to lead”, as Hillary Clinton said in her moving concession speech (why couldn’t she have sounded like that once or twice during the campaign? But that’s another story).