There is “an enormous gap in the centre of British politics”. James Chapman is by no means the first person to assert this, or to hope to fill the void with a new party.

Winston Churchill always chafed under the constraints of the two-party system. So had his father, Lord Randolph Churchill, and so did his father’s great friend, Lord Rosebery, a Liberal of glittering abilities who in 1894-95 had proved a complete failure as Prime Minister, though his horses won the Derby in both those years.

Churchill saw much of Rosebery at the start of the 20th century, and later wrote a brilliant account of their friendship:

“He was out of sympathy with the Liberals: I was soon quarrelling with the Tories. We could both toy with the dream of some new system and grouping of men and ideas, in which one could be an Imperialist without swallowing Protection, and a social reformer without Little Englandism or class bitterness. We had certainly that solid basis of agreement and harmony of outlook upon middle courses, which is shared by many people and was in those days abhorrent to party machines. Need one add that the party machines always prove the stronger?”

The Tory Party came very close to being wrecked by Tariff Reform, or Protection as Churchill called it, and suffered a catastrophic defeat in the election of 1906, by which time he had jumped ship and joined the Liberals.

Today the great issue which could wreck the party is Europe. Here is the motive which yesterday impelled Chapman to denounce his former employer, David Davis, as “a sexist lying lazy incompetent boor”.

Such language, though normal on Twitter, may make some people question how closely they want to work with Chapman. But when I knew him in the Commons press gallery, he was an affable, industrious and trustworthy individual. And like the rest of us, he is entitled to challenge the Government’s line on Europe.

The main reason why one might hesitate to join the Democrats, as he intends to call his new party, is that in British politics, such ventures almost always fail.

No new party has won a British general election since Labour in 1945. By then, two generations after it was founded, Labour could no longer be described as entirely new: its leading figures had been pillars of Churchill’s wartime coalition, which was one reason why they were trusted by the voters.

In this age of social media, it is a thousand if not a million times easier to form a movement than a party. A movement can spring into existence in a few hours, as Chapman’s did. The almost impossible bit is to make the transition to a government in waiting, with a leader and other prominent figures who command popular trust, and a credible manifesto.

A campaign on a single issue is one thing. A programme for government requires many things, and indeed many unsatisfactory compromises. It demands a large number of committed people who are prepared to work together, even though most of them will give their time for free, none is going to get everything he or she wants, and only one at a time can be leader.

It requires too a permanent, professional staff of organisers, who are constantly adapting themselves to new methods and circumstances. This organisation has to be resilient enough to survive defeats, sometimes for several general elections in a row, and to learn lessons from these. It soon loses the charm of novelty and the advantage of surprise, and cannot expect to live for more than a month or two on a diet of easy victories won over opponents caught off balance.

On the contrary: the main effect of any successful new movement is almost invariably to force the existing party machines to raise their game, and to steal any popular elements in the insurgency. The Conservative Party has been doing this since the 1830s, and Labour since it supplanted the Liberals in the 1920s.

In countries with a directly elected President – the United States, say, or France – a new movement can hope in a burst of popular enthusiasm to seize power at national level. In a long-established parliamentary system, such as Britain’s, this is almost impossible, for the Prime Minister must command the confidence of a majority of MPs, which means he or she must lead one of the major parties.

For the past year, I have been writing a volume of brief lives of all 54 British prime ministers since 1721, which will appear next year. Quite a few of them have been failures, but none has been such a disgusting oaf as Donald Trump. It is inconceivable that the Commons would allow such a character anywhere near Downing Street. Here is a constitutional safeguard against the abuse of power which is not often noticed, but works very well.

Nigel Farage is far less objectionable than Trump. But the man who did more than anyone else to bring UKIP to public attention has not even gained election to the Commons, let alone become Prime Minister.

UKIP aspired to be a party, but turned out to be no more than a campaign, whose main demand – an EU referendum – was stolen by David Cameron. That is a remarkable achievement for Farage, and one which I guess Chapman would be delighted to emulate, even if it takes him a quarter of a century to get there.

But anyone who hopes to become an MP, and therefore eligible for high office, will continue to regard the two main parties as a much better bet. And this will severely limit Chapman’s ability to attract good recruits. Even before its recent troubles, UKIP had difficulty finding candidates whom it was possible to imagine as Cabinet ministers.

The Social Democratic Party, founded in 1981, was led by four Labour politicians who were all quite clearly of Cabinet calibre. Roy Jenkins, Shirley Williams, David Owen and Bill Rodgers were a gifted gang, who recruited over two dozen Labour MPs and a membership full of idealistic members of the educated middle class who were fed up with the existing parties and were for the most part new to politics.

Bliss was it in that dawn to be alive. After the humiliations of the 1970s, when both Labour and the Conservatives had failed to surmount Britain’s problems, it seemed to SDP supporters that their new party had found the answer. It rose to 50 per cent in the polls and won some early by-election triumphs.

But although Margaret Thatcher, the Conservative Prime Minister, was at this point deeply unpopular with many of her own MPs, only one of them, Christopher Brocklebank-Fowler, decided to cross the floor and throw in his lot with the new party. Alan Clark recorded the scene in his diary for 16th March 1981:

“Brocklebank-Fowler staged his defection today… Poor nice Roddy MacLennan [a Labour MP who had joined the SDP] got up to make way for Brocklebank. The whole House roared with laughter, which was not the reaction intended by the Social Democrats.”

And although the new party had high ideals, it soon faced the agonising business of forming some sort of electoral pact with the Liberals, who were naturally displeased to find a new force marching into the centre ground where they for so long had maintained a precarious existence.

Why had the SDP MPs not joined the Liberals? The answer is that in some cases, they detested the Liberals even more than they detested the left-wingers, hostile to Europe and in favour of unilateral nuclear disarmament, who had taken over the Labour Party.

At the general election of 1983, the SDP-Liberal Alliance won 26 per cent of the vote, only two percentage points behind Labour. But the first-past-the-post electoral system gave Labour 209 seats, while the SDP-Liberal Alliance had only 23.

This split in the Opposition vote was of huge benefit to the Conservatives, though in the long term one could contend that it also benefited Labour, who in 1997 won a landslide victory after adopting pretty much everything the SDP believed in.

Why don’t Chapman and his supporters just join the Liberal Democrats? After all, the Lib Dems agree with them that Brexit is going to be a disaster.

And Vince Cable, the new Lib Dem leader, is a considerable figure, who understood the need to nationalise Northern Rock quicker than another of Chapman’s former employers, George Osborne. One can be pretty confident Cable will extend the hand of friendship to any Labour MPs who decide they can no longer stand Corbyn, perhaps because he has failed to condemn military action by Putin against the Baltic states.

Cable will also welcome any Labour MPs who find themselves purged by the Corbynistas, a scenario which is perhaps even more plausible.