Second preference votes could prove crucial in the race
Liberal Democrat mayoral candidate Brian Paddick has warned his rivals he will not be endorsing either - in the battle for voters' second preferences.
Labour's Ken Livingstone said he shared "90%" of policies with the Lib Dems and once urged Labour voters to back a Lib Dem MP in a marginal seat in 2001.
But Mr Paddick, who polls place third, said it was a "naked attempt" to get his second preference votes.
Tory Boris Johnson said he did not want to be BNP voters' second preference.
If none of the candidates in the race to be London mayor gets more than 50% of votes at the first count, all but the top two candidates are knocked out and their second preference votes shared out.
Polls suggest the contest is neck and neck between the Labour incumbent, Mr Livingstone and his Conservative challenger Boris Johnson.
A Sunday Times telephone survey of 1,000 Londoners put Mr Livingstone on 45%, narrowly ahead of Mr Johnson on 44% while former Metropolitan Police commander Mr Paddick was on 9%.
The Green Party candidate Sian Berry and Left List candidate Lindsey German have both urged their backers to put Mr Livingstone as their second choice.
In the previous two London mayoral elections, Mr Livingstone had to go to the second round to secure victory.
In a special edition of BBC One's Question Time on Thursday, the mayor said: "We're all fighting for the heart and soul of Brian Paddick to come out either for Boris or me because that's the real case Londoners face."
He said he had urged Labour voters in Kingston and Surbiton to back the Lib Dem MP Ed Davey in 2001 - to keep the Tory candidate out.
"I actually think Lib Dems and myself agree on 90% of policy," Mr Livingstone said, citing Lib Dem support for a £25 congestion charge on gas guzzling vehicles.
But Mr Paddick replied: "I don't think that's true Ken ...this is a naked attempt for you to try and win second preference votes from me."
"Absolutely," said Mr Livingstone. "Of course I want the support of Lib Dems."
But Mr Paddick, who was approached by the Conservatives to run for them as mayor but turned them down, replied: "What I'm saying to Lib Dems is you're as bad as each other, you're both wrong for London, don't vote for either of these people."
"My job as a candidate is to say to people, I am the best candidate, vote for me first preference, what you do in terms tactical voting to get rid of either one of these two is a matter for you."
Mr Johnson said: "I would not be so presumptuous as to tell any voter which way to direct their second vote."
The British National Party are said to have told their voters to put "the joker Boris" as their second preference.
But Mr Johnson told the programme: "I don't want a single second preference vote from a BNP supporter."