Brutal history lesson for Lib Dems

But their fate is a warning about the perils of coalition for Nick Clegg, who could go down in history as the leader who took his followers to the promised land of electoral reform, or the man who allowed his party to be swallowed up by the Conservatives.

At the end of the 1920s, the Wall Street crash plunged Britain into economic crisis. As trade collapsed and credit dried up, unemployment soared.

The Labour minority government, already reliant on Liberal MPs to pass any legislation, struggled to find a way to balance the books that was politically acceptable and tough enough to avoid a massive deficit.

It was forced to resign and a National Government was formed, bringing together Conservative and Liberal MPs under the leadership of the Labour Prime Minister Ramsay MacDonald.

He warned the nation that everyone must "bear equitable sacrifices to see the present crisis through" but for the Liberals, the sacrifices came at a much higher price than that paid by any of their coalition partners.

"The Liberals' decision to join the National Government in 1931 ended up being almost terminal in terms of their electoral fortunes," says the historian Dominic Sandbrook.

"Within just a few weeks the Liberals were falling out among themselves. They were punished at the polls and they virtually disappeared so that by the early 1950s, there were just five MPs left.

"It was said, and it was completely true, that the Liberal parliamentary party could hold their meetings in the backseat of a taxi."

The coalition deal split the Liberal Party in three as MPs unhappy with Conservative policies, and dissatisfied with the opt-outs negotiated by their leaders, railed against the agreement.

Amid the infighting, the National Government prepared an emergency budget that slashed public sector wages, put up income tax and cut unemployment benefits by 10%.

Then, as now, the Liberal MPs provided the Conservative leader, Stanley Baldwin, with convenient political cover for the cuts.

"Baldwin saw the advantage of sharing responsibility for what were bound to be unpopular policies," historian David Dutton says.

"He used the presence of roughly three dozen Liberal MPs within the government as a means of sharing responsibility for the cuts the government had to introduce to balance the budget."

As the Tories made sure much of the blame for the cuts was heaped on the Liberals, they also undermined the political identity of their coalition partners.

"Stanley Baldwin was a kind of David Cameron in prototype because what Baldwin wanted to do was to squeeze the Liberals and to reposition, to effectively rebrand the Tory party as a party of the common ground, of the centre and a party of Middle England," Mr Sandbrook said.

"Baldwin succeeded triumphantly. He turned the Conservative party into the most successful vehicle for wielding power in British history."

Liberal leader Herbert Samuel was left at odds with his party. As the 1935 general election approached, Mr Samuel appealed to once-Liberal constituencies not to abandon them.

"I honestly hope that in every such constituency those who care for Liberalism but doubt whether the Liberal candidate will be elected, will vote solidly for Liberal candidates," he said in an election broadcast.

"We may achieve some surprising results."

The only surprise in the election result was the degree of the Liberal catastrophe.

Mr Samuel and two-thirds of Liberal MPs lost their seats, and the party plunged into 20 years of near total irrelevance.