Nick Clegg has always said that if no party can command a majority in parliament, he will support the party voters have been seen to support. It was assumed that if the Tories were the biggest party - but a few members short of an outright majority - he would back them.
But if the YouGov poll comes true, on a uniform swing Labour will be the largest party and the Liberal Democrats still the third largest party in parliament but with 90-odd seats. With Labour out front, Clegg's logic would suggest he would back a Labour-led government. But Clegg's poor relationship with Gordon Brown is well known and with such a strong showing for the Lib Dems will he feel less bound by who is the largest party and feel empowered to bargain more freely? Will he feel mandated to change his view mid-way through a parliament? Will he be willing to hang the threat of early elections over the head of the British prime minister?
The TV debates have changed British politics for ever. So much is clear. But the change may be more profound than many realise. It could have ushered in a continentalisation of British politics - with the Lib Dems in a semi-permanent "King-maker" role. Nick Clegg may become Britain's version of Hans-Dietrich Genscher.