One of the most notable trends in the opinion polls at the moment is the surge in support for the small parties. Today's YouGov/Sunday Times poll puts their combined support at 16 per cent, the highest level since summer 2009. UKIP, which polled 3.1 per cent at the last election, is on six per cent, the SNP and Plaid Cymru are on 3 per cent (YouGov doesn't provide individual figures) the Greens are on 3 per cent and the BNP are on 2 per cent.

The major explanation for this development is the entry of the Lib Dems into government, which has left them unable to compete for the protest vote. Their supporters have mainly defected to Labour (backed by 37 per cent of 2010 Lib Dem voters) but also to UKIP (backed by 6 per cent) and to the Greens (backed by 8 per cent).

Ahead of tomorrow's vote, the growing support for UKIP, which cost the Conservatives up to 21 seats at the last election (there were 21 constituencies in which the UKIP vote exceeded the Labour majority), will trouble Tory strategists. The government's decision to whip MPs against an EU referendum represents a major political opportunity for Nigel Farage's party. For the first time, at the next election, it will be able to attack Tory MPs for explicitly voting against an in/out referendum.

Mischievously, in an interview with the Sunday Telegraph, Farage suggests that UKIP may give pro-referendum Tories a free run at the next election. He tells the paper:

I'm not giving any blanket assurances but we would certainly look far more favourably at those who would put their consciences and their constituencies ahead of the whips.

Given that the presence of a UKIP candidate can make the difference between winning and losing, Farage's words will only increase the likelihood of 70+ Tories rebelling tomorrow.