By attracting disgruntled Conservative voters, Referendum Party candidates could let Labour in a number of constituencies. So, although most Referendum supporters are Euro-sceptics, their votes are likely to strengthen a Labour government that will be more pro-European than the Tories.
Independent/Harris polls through the campaign have put the Referendum Party vote on an average of 2 per cent. And analysis of the make-up of Sir James's support over four weeks suggests that it comes overwhelmingly from people who would otherwise be Tories. Of Referendum Party supporters who voted at the last election, more than 80 per cent say they voted Tory.
According to our figures, the Referendum Party could steal as much as 1 per cent of the electorate from the Conservatives - enough to hand an extra six seats to Labour and increase Mr Blair's majority by 12.
In Putney, south-west London, where Sir James Goldsmith himself is standing, strong support for the Referendum Party could split the Tory vote and lead to a Labour win. David Mellor, the Conservative candidate, won with a majority of 7,526 in 1992. Since then, the threat of the Referendum Party, plus the damage caused by Mr Mellor's resignation in 1993, could mean that the former minister fails to get back in.
According to Dennis Meehan, agent for the Labour candidate Tony Coleman, their canvassing returns indicate that the Referendum Party is an unknown quantity in the constituency, but he suspects that Sir James's support could be fairly substantial. This week's backing of The Times for Sir James in Putney may also have a marginal effect in deflecting votes from Mr Mellor to him.
Across the rest of the country, the Referendum Party is not likely to have such a dramatic effect. In Reigate, Surrey, even the support of The Times for the candidacy of the maverick Euro-sceptic Sir George Gardiner following his deselection by the local Tory party is unlikely to enable him to help Andrew Howard, the Labour candidate, defeat the Conservative Crispin Blunt (former adviser to Malcolm Rifkind, the Foreign Secretary) who is sitting on a notional 16,940 majority.