One of the reasons why some Conservatives believe it will be impossible [2] for David Cameron to win a majority at the next election is the scale of the defection of Liberal Democrat supporters to Labour. If Ed Miliband's party hangs on to around a third of the Lib Dems' 2010 voters, the Tories stand to lose dozens of seats at the next election - there are 37 Conservative-Labour marginals where the third place Lib Dem vote is more than twice the margin of victory. In the 1980s, it was the formation of the Social Democratic Party and the resultant split in the centre-left vote that allowed Margaret Thatcher to win successive landslide victories. In 2015, the collapse in support for the Liberal Democrats and the reunification of the left around Labour could bring Miliband to power.

This fact has led some Conservatives to wonder aloud whether a change of Liberal Democrat leader before 2015 is now in their interests. The hope is that a more left-wing leader such as Vince Cable or Tim Farron, both of whom have signalled their availability [3], could prompt the party's former supporters to return home from Labour. ConservativeHome editor Tim Montgomerie recently told me [4] that "a left-wing replacement" of Nick Clegg in 2014 was "vital to Tory hopes".

Those with an interest in a Lib Dem recovery have been encouraged by polls showing that the party would perform better with Cable as leader. A ComRes survey [5] last September showed that support for the Lib Dems would rise to 18 per cent under Cable, compared to 14 per cent under Clegg. However, it is doubtful whether this bounce would last once Cable was forced to take responsibility for all coalition decisions (something he has skillfully avoided doing to date. Few would know, for instance, that it was Cable's department that introduced higher tuition fees) It is also the case that the party's former left-wing supporters, those who defected from Labour over Iraq and top-up fees, are likely to prove the hardest to win back.

But as we get closer to the election, this discussion will be had with increasing frequency in Lib Dem and Tory circles. If the Lib Dems are still flatlining at 10 per cent in the polls in 2014, it is hard to see the party not taking a gamble on an alternative leader. The dilemma for the Tories is whether to help shore up Clegg's position, for the sake of coalition unity, or to tacitly encourage a revolt against him.