I know a thing or two about new parties. I joined the SDP as a founder member a few days after my 18th birthday in 1981. I was a councillor, activist and parliamentary candidate for the SDP and its successor party, the Liberal Democrats, for 14 years before joining Labour when Tony Blair became leader and abolished Labour’s old clause IV – committing to general nationalisation – in 1995.

The SDP’s founder, Roy Jenkins, was my hero and later mentor. The most transformational liberal home secretary ever, he legalised both homosexuality and abortion in one of the most skilful ministerial manoeuvres of parliamentary history. Roy was a standard bearer of a generation of pro-Europeans, Britain’s only president of the European commission, and a brilliant biographer. I read his awesome Asquith at the age of 14 and still regard him as the best prime minister Britain never had. There is no potential leader of a new party today with remotely comparable credibility.

But in retrospect the SDP was born to fail. Roy would have done better simply to have joined the Liberals and become their leader in 1981. In its only serious contest – 1983 – the SDP, in alliance with the Liberals, did only marginally better than had the Liberals in their Jeremy Thorpe-led surge in 1974. It was rapidly downhill thereafter as Labour regrouped under Neil Kinnock and John Smith.

“Breaking the mould” – the SDP’s slogan – is hellishly difficult in Britain’s first-past-the-post parliamentary system because the hurdle is so high. Ignore the apparent lessons of Macron – the French have a presidential system, and he had the luck of the gods. Ignore Labour’s rise – that took 50 years, two world wars, a split in the Liberal party, a disastrous coalition between half of the Liberals and the Tories; and even then the Liberals refused to die and Labour has only infrequently, and fairly precariously, held power since the 1920s. Ignore too Scotland, where the SNP – another party with a 50-year rise – is a regional force in a nation where the Tories have historically always been weak.

Focus on just one thing: the Conservative party in England. Unless the Tory party splits – which it has done but once in the last 200 years, over the repeal of the Corn Laws 170 years ago – a new party is doomed because the Tories are England’s dominant nationalist party and a supremely successful cultural and electoral behemoth. Consider. Since 1900 the Tories have been in office for 78 of the 118 years. In that time there have been 14 Tory prime ministers, three Liberal and six Labour – and two of those Liberals and one Labour ended up heading largely Tory governments and were destroyed by them.

Labour and the Liberals have never joined in a coalition without the Tories also in the mix. Since 1945 the Liberals have been in office once, with the Tories, and that consumed Nick Clegg and nearly his party too.

For those toying with a centre party because of Brexit – Brexit is happening because of an extreme English nationalism unleashed by the Tories’ most dominant leader since 1945, Margaret Thatcher, and nurtured by disciples including Nigel Farage. Split the opposition and Brexit is certain. The Conservatives are by far the most successful democratic party in Europe, apart possibly from Sweden’s social democrats, because they have synthesised democracy and nationalism to a unique degree in Europe’s historically largest homogeneous nation, England.

There are very deep social roots. England’s dominant schools, universities, professions and enterprises are largely in the ideological and filial grip of the Conservative party. This isn’t always obvious but it is emphatic, especially when they are threatened. In England, new parties don’t work, but reverse takeovers of existing parties sometimes do – Thatcher of the Tories, Blair and Corbyn of Labour, Clegg and his orange-bookers of the Lib Dems. Ukip is essentially a Farage reverse takeover of the Tories, so successful that I would not bet against Farage yet becoming a Tory MP and the party’s leader.

The Conservatives have been unusually badly led by David Cameron and Theresa May. But May will be gone soon. Unless the Tory party splits – and I see no sign of it – a new party will almost certainly fail to break through.

Roy Jenkins understood this. He told me after 1983 that the failure to attract Tory MPs – only one joined – was the SDP’s crucial weakness. Ironically one of his closest friends was the leader of the Tory wets of the 1980s – Sir Ian Gilmour – but Gilmour was a baronet, his wife had been a bridesmaid of the Queen, and it was socially impossible for him to leave the Tories. It is the same today with Tory “wets” such as Tom Tugendhat, Philip Hammond, Dominic Grieve and David Lidington.

A footnote. When I fell politically in love with Tony Blair in 1994, I was inconveniently a Lib Dem parliamentary candidate. Anguished, I asked Roy what I should do. Without a moment’s hesitation he said “join Labour. Tony is Asquith. He may be Attlee too. At your age, join the large social democratic party not the small one, and stick to it.”

That’s what I’m doing.

• Andrew Adonis is a Labour peer