In his seminal book The Progressive Dilemma, the historian David Marquand lamented the splitting of the centre-left vote in Britain.
The existence of both the Liberal Party (later the Liberal Democrats) and Labour had ripped asunder the country's centre-left majority, bequeathing a Tory-dominated 20th century.
This split was painfully evident during the recent referendum on electoral reform. Though Labour joined the Lib Dems in proposing a switch to the alternative vote, a seemingly endless procession of Labour dinosaurs made the conservative case for retaining the current system. This was probably crucial in dashing hopes for change.
Marquand's analysis is right about the past century. Three-party politics scarred the centre-left. What's interesting about Westminster's current configuration is that four-party politics is on the way, and will probably scar the centre-right.
Ukip – a "libertarian, non-racist party seeking Britain's withdrawal from the European Union" – is the product of civil war in British conservatism during the 1980s, one victim of which was Margaret Thatcher's reign as Prime Minister.
Europe divided Thatcher's Tory Party into a soft, modernising faction and a harder, resistant one. The modernisers won the Tory Party. The resistant bunch deserted it. As the writer Peter Oborne put it recently, "Ukip is in reality the Conservative Party in exile". That is one reason why it now regularly achieves 7 per cent in national polls.
Ukip has policies far beyond Europe, and its members are generally patriotic, pinstriped types. Their views on immigration are stupid and myopic, but that is true of much of the public. As the whole European federalist project disintegrates, Ukip will appear prescient, and voters will give them credit.
This will damage the Tories in two ways. First, by depriving them of crucial votes in marginal constituencies; and second, by pulling the Tories away from the centre ground, where elections are won.
It follows that anyone who thinks of themselves as centre-Left, including all Labour and Lib Dem voters, has an enormous incentive to strengthen and promote Ukip. I have a feeling that, disgruntled at the manner of his departure from the Cabinet and the tedium of backbench life, Liam Fox might fancy switching to Ukip if offered its leadership.
Though barely noticed, the progressive dilemma has acquired a tenacious sibling across the political divide. In the 21st century, the conservative dilemma will be disastrous for conservatism.