Last night a dozen senior party figures, including Simon Hughes, the Liberal Democrat president, were making public their opposition to Sir Menzies’s policy to abolish the proposed 50p top rate of tax for the rich.

A defeat this afternoon in Brighton would be a huge embarrassment and overshadow Sir Menzies’s first Liberal Democrat conference as leader.

The issue has exposed the split between the left and right wings of the party, amid concerns among activists that Sir Menzies is trying to shift the party to the right.

Sir Menzies admitted last night that he was worried about the outcome of today’s vote, while Steve Webb, the health spokesman, told the Times fringe meeting that the party had found itself in a “crazy situation”.

Vince Cable, the Treasury spokesman, said at the same event that the party would be damaged if the leadership were defeated.

Evan Harris, a party spokesman, who is leading the revolt, was rallying support for his amendment to keep the proposed 50p top rate of tax for people on £150,000 a year.

His supporters include Sandra Gidley, John Leech, Norman Baker, Mike Hancock, Phil Willis, David Howarth and Bob Russell from the parliamentary party and Baroness Tonge. Baroness Williams of Crosby, who is one of the party’s most prominent figures, made her opposition clear in a newspaper article, saying that the party “must be a party committed to fairness and that has to mean some redistribution”.

But in a sign of the party’s nervousness over today’s vote, she has told friends that she has decided against speaking in the debate because she accepts that the row over the tax policy has become an issue about Sir Menzies’s leadership.

The most damaging intervention came from Mr Hughes, who pointedly refused to endorse the plans, adding that it was “not the end of the world” if party members defied Sir Menzies.

He claimed that he was not taking sides in the debate because of his role as president — but his lukewarm words will have infuriated allies of Sir Menzies.

He said: “My colleagues know that I’m not going to participate in the debate. I explained to them and it was accepted that as president that was the right thing to do.

“My message to the party is listen to the debate and, if you are persuaded, vote for the new proposals. If the proposals go down, they go down. It’s absolutely not the end of the world,” he told the London Evening Standard.

But party enforcers issued a “clarification” on behalf of Mr Hughes, which quoted him as saying that the “party will have a policy to be proud of, if it is passed unamended”. But the statement again stopped short of personally supporting the measures.

The issue was one of the themes at the Times fringe meeting. Mr Webb, who is on the left of the party and has previously declared his doubts about the policy, said that he would back Sir Menzies in today’s vote after being persuaded to support it by colleagues.

He also played down the consequences of defeat. “If it doesn’t (succeed), I hope we can say we’re a big enough party to cope,” he told the meeting.

Meanwhile, Mr Cable, who will attempt to take on critics in this afternoon’s debate, told the meeting that the Liberal Democrats should not always automatically favour higher taxes. “We’re now in an era of fairer, not higher, taxes. There is a specific debate about the 50p rate. Many countries don’t have very high marginal tax rates,” he said.

“If it’s defeated I don’t know what will happen. It will be damaging, but we will accept the views of conference.”

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