Senior NSW Liberals said it would take a "miracle" to win the Wentworth byelection, labelling a damaging email smearing independent Dr Kerryn Phelps "the final nail" in the campaign.

Sources close to the campaign said the email which claimed Dr Phelps had HIV and was pulling out of the race was "hugely damaging", and had cruelled their chances of holding the seat.

Liberal candidate Dave Sharma called the email "vile" and supported Dr Phelps' demands for it origin to be investigated by the Australian Electoral Commission.

But party sources said the email, alongisde the leak of the Ruddock report into religious freedom, would have the desired effect of causing "maximum damage" and portraying the Liberals as "homophobes".

In a last ditch appeal to disillusioned Liberal voters, former Prime Minister John Howard on the hustings in eastern Sydney's Double Bay on Thursday, in a bid to drive home the message that a strong protest vote would have dire consequences for the federal government.