Two of the great political parties in the west — the Republicans in the US and Labour in the UK — are in a state of near collapse. That, in turn, threatens the health of democracy on both sides of the Atlantic.

The crises in the Republican and Labour parties are strikingly similar. In both cases, a leader has emerged from the fringes of politics and taken the party in a different and radical direction. The emergence of Donald Trump and Jeremy Corbyn threaten to destroy the electoral prospects of their two parties — and will sow division and ideological confusion long into the future.

Even if Mr Trump and Mr Corbyn never make it into the White House or 10 Downing Street, their ascendancy is also damaging to the wider political system. Well-functioning democracies need a credible opposition to hold the government to account. But in the UK and the US, that basic function is no longer being properly performed.