Before the last election Lib Dem voters were split between preferring a coalition with the Tories or a coalition with Labour. Since then they have lost most of their Labour leaning supporters. This is probably why Labour now have a solid lead in the opinion polls despite the disastrous performance of the last government and the widespread lack of faith in Ed Miliband.
This isn't to say the Lib Dems made the wrong decision in May, had they gone into coalition with Labour then their more right leaning supporters would have left instead.
What this means though is that Labour will do much better in bye elections, such as the one yesterday in Oldham, between now and 2015 than had previously been anticipated and could be a much more substantial force at the election than previously believed.