By this time tomorrow, the anti-EU United Kingdom Independence Party is likely to be celebrating its first member of the Westminster parliament. Polls have just opened in the deprived seaside town of Clacton where the sitting Conservative lawmaker switched to UKIP and called a vote.
A second member of the ruling Conservative party has now defected to UKIP and will force another by-election before long leaving the party on tenterhooks over who might be next. Many fear they will lose their seats at the May 2015 general election as UKIP splits their vote.
Could that be enough for them to turn on David Cameron? Maybe not but if they did go for a new leader they would inevitably want someone who was more anti-European with all the implications that would have for a planned referendum on EU membership in 2017.
Cameron has a strong argument – that only by keeping him as prime minister will Britons get an in/out EU referendum. To vote UKIP and split the right-wing vote would be to give the opposition Labour party the keys to power. It has not promised an EU vote.
But while some recent opinion polls put the Conservatives marginally ahead, the average of polls puts Labour about four points up.