The Anglo-French millionaire's party threatens to stand, on the single issue of Europe, in any seat where no main party candidate has pledged a referendum on membership of the European Union.
Sir James's interview on BBC1's On the Record on 3 December worried some Tory MPs, and increased the pressure on John Major to promise a referendum on the single European currency. Sir James insisted Tory candidates would have to commit themselves to a referendum on the Maastricht treaty itself to avoid having a Referendum candidate against them - but many feel the simple promise of a referendum on the single European currency would satisfy potential voters for Sir James's party.
Colin Rallings, of the University of Plymouth, said yesterday: "They are right to be worried. To hold on at the next election, the Conservatives have to do as well as they did in 1992, and Labour have to do as badly, and the Referendum Party could still stop the Conservatives winning."
He calculates that if Sir James's party wins 1.5 per cent of the electorate, two-thirds of whom would otherwise have voted Tory, the Tories would lose 11 or 12 seats to other parties.
"Our assumptions about the prospects for the Referendum Party are extremely modest - but fair if you look at the history," Mr Rallings said. Green Party candidates won an average 1.8 per cent of the vote at the last election.
"The amount of money he says he'll spend could make a lot of difference in some constituencies," David Shaw, the Euro-sceptic Tory MP for marginal Dover, said yesterday.
Sir James is reported to be prepared to spend pounds 20m on his campaign - close to matching the spending of each of the two main parties. He is believed to be ready to employ a polling company and an advertising agency. He would also qualify for a party political broadcast.
The Referendum Party held a training session for candidates last weekend, although so far it has only two big names: Sir Alan Walters, Baroness Thatcher's former economic adviser, who will challenge the Chancellor, Kenneth Clarke, for his Rushcliffe seat, and Sir James himself, who has not decided where he will stand.
After last week's interview, the BBC logged 20 calls from members of the public in support of Sir James or inquiring about how to join his party.
In the interview, Sir James said: "The people of this nation voted to go in in 1975 on a false prospectus ... I want them to have the right to vote ... if they are not given that choice and the power is transferred, that in my view is tantamount to treason."
t The growing Conservative consensus behind a pre-election promise of a referendum on a single European currency is underlined today by public backing for the idea from a former senior adviser to Douglas Hurd.
Speculation that John Major will promise not to join a single currency without a referendum is heightened by the call from Michael Maclay, writing in this newspaper, for a "populist and principled" pledge to let the people have the final say.
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