The crowning of Boris Johnson as Prime Minister suggests that UK politics will end up as extreme as in the US As potentially the worst prime minister of my lifetime takes power, it will be fascinating and frightening to observe Westminster

As a Republican President spews out racist bile against four ethnic minority women Democrats, it is easy to forget the transformation of United States politics in recent decades.

When I was born in 1962, southern Democrats defended segregation, while there was a proud strain of liberal Republicanism that sought to use the state for progressive ends.

Their standard-bearer was Nelson Rockefeller, governor of New York, who transformed higher education, created a huge wilderness park and fought developers such as Donald Trump’s father Fred to stop housing discrimination. Martin Luther King once said that if the south had leaders like “Rocky”, inequality might have ended in months.

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Rockefeller was his party’s favourite for the presidency in 1964, but the billionaire’s run was stymied by a messy recent divorce. He tried again in 1968, seeking to heal wounds of a nation torn apart over the Vietnam War, but lost out to Richard Nixon at a tumultuous convention in Miami.

Then Nixon resisted pressure from aides to pick as his running mate John Lindsay, the mayor of New York city who later joined the Democrats. The moderates were crushed in Miami – and although no-one knew it at the time, this was the moment the Republicans turned away from liberalism.

A late run by Ronald Reagan was also thwarted at that convention. Twelve years later, he won the South on his way to the White House. Since then, his party has slid from Reagan’s sunny optimism through the Tea Party into Trump’s hate-fuelled Nativism. Now it is led by an overt racist.

The Berlin-based Manifesto Project, which analyses party policies, suggests the Republicans have become more extreme than both UKIP and the far-right National Rally (formerly the National Front) in France. This reminds us that political parties can move around the political spectrum.

The big question for Britain is whether we are seeing a similar situation as the Conservative Party, in thrall to a nationalist sect once dismissed as a nuisance, prepares to crown a divisive populist, and Labour digs in under its hard-left leader despite the stench of anti-Semitism.

Last week alone we saw Gloria De Piero stand down as shadow justice minister and say she was quitting parliament, hitting out at ‘lack of tolerance’. Then former chief whip Hilary Armstrong was expelled from her Durham branch after criticising Jeremy Corbyn. ‘This sadly shows how far the party has departed from its roots,’ she said rightly.

Both parties have shifted their terrain – highlighted by fragmentation that has seen a handful of MPs already depart both sides in disgust. Labour looks lost to the hard left when even an official inquiry into anti-Semitism, compounded by humiliating contortions over Brexit and dismal failures to land blows on a hopeless government, fails to shake the party enough to sack its leader.

Even an election triumph would only make matters worse by putting Jeremy Corbyn more strongly in the spotlight to expose his lack of intellectual heft, cabal of intolerant aides, crass anti-Americanism and regressive, free-spending socialism.

Now the Tories are doing their best to demean themselves by – almost certainly – imposing Boris Johnson on the country as Prime Minister. Never mind that he appears to be a serial liar, slippery, lazy and incompetent – a man who should not be trusted to open the Downing Street door, let alone given the keys.

Some still fall for the idea this self-serving chancer is a liberal despite his embrace of nationalism and targeting of Islamic women, yet in terms of policy in the depressing leadership campaign he has pushed tax cuts for the rich, pushed tired war-on-drugs rhetoric and pushed the termination of sensible prison reform while letting his White House admirer oust a British diplomat.

Johnson talks of desiring unity, having done so much to widen divisions by leading this country blindly into its current morass. His hazy plans for Brexit have spread such fear that the Pound has fallen again. Sensible cabinet ministers are queueing up to quit over being forced to back the disastrous idea of Britain leaving the European Union in three months time without a deal.

‘As potentially the worst prime minister of my lifetime takes power, it will be fascinating and frightening to observe Westminster over the weeks ahead’

Consider the state of politics when the chancellor Philip Hammond, emerging as an unlikely rebel leader, admits he did not tell his boss he was abstaining on a critical three-line vote in order to restrain her successor from suspending parliament to drive through the hardest possible Brexit.

Our new prime minister will spout the standard platitudes of hope and glory on the steps of Downing Street this week. But as tensions over Iran fuel the possibility of global recession, we stand on the brink of a tempestuous period in British politics watching an outmoded system struggle to handle torrid forces unleashed by Brexit.

The Tories are hurtling to the right to see off Nigel Farage’s new force, while driving away the younger generations and liberal voters they will need in the future. Johnson’s aides are courting Labour rebels on Brexit while principled moderates on all sides team up to thwart his incoming government.

Meanwhile Remain parties have joined forces for a by-election that could shrink the Government’s tiny majority still further. The kaleidoscope has been shaken and the pieces are in flux.

As potentially the worst prime minister of my lifetime takes power, it will be fascinating and frightening to observe Westminster over the weeks ahead. Anything is possible- from crashing out of our key trading alliance without a deal through to Corbyn winning power after a general election.

Whatever happens, historians may look back at these tempestuous times and see it as the point of political realignment – just as that Miami convention half a century ago marked the moment the Republicans started sliding to the hard-right, leading to sickening appeasement of a racist President.