Ken Livingstone will know in a little over three weeks whether he will get back his old job of London Mayor, but with recent polls suggesting he will not be returning to City Hall, figures on the left have begun issuing desperate pleas to the people of London - pleas which should be ignored.

Supporters of Livingstone, who has been heavily criticised in recent weeks for apparently taking steps to avoid income tax and for some ill-judged comments about London's Jewish community, have been reduced to asking Londoners to vote for their man on the grounds that he is not Boris Johnson.

Mehdi Hasan said in the Guardian yesterday that "if you don't want to see Boris re-elected then you have to vote for Ken," a sentiment that former Labour MP Tony McNulty agreed with by adding on Twitter that "all else is indulgence".

To his credit Hasan is not playing the part of an unthinking party loyalist - he has acknowledged that Livingstone's tax arrangements were perhaps less than ideal - but still that does not excuse deploying the second worst argument in political debate today that we should vote for A in order to keep out B.

When politicians start telling you to vote for them because of who they are not, then you should know at once (as if you did not already) that they have nothing positive to offer and that their only real interest is themselves.

It is not only the Labour Party which is guilty of this. At the last election the principle-free Tories told us we had to vote for them to get Gordon Brown out or the country would sink into a sea of debt (isn't debt still going up by the way?). It was also said by people on the left that it would be rather good to vote Lib Dem in order to "keep the Tories out".

The case of the Lib Dems is of course a perfect example of why no one should ever vote for an undesirable candidate just to keep out an even more undesirable candidate.

More than that the "Vote for A to avoid B" school of thought strikes at the very heart of what it means to live in a democracy. We should be free to vote for what we believe in rather than allow ourselves to be corralled like cattle by politicians into voting for them when we don't really want to. This is not North Korea for goodness sake.

Having lived in Dagenham I know a little of what it feels like to be confronted with a choice between bad and badder. In Dagenham I did give some serious consideration to voting Labour, a party I cannot stand, just to kick out the BNP who at the time were the official opposition on the council and who would no doubt disapprove of my foreign wife.

In the end I decided against it. If democracy is to mean anything, it should mean that I can vote for what or who I believe in. Why should I be pressured into voting for a Labour councillor whose principles I despise and who in all likelihood can barely even read out loud?

Ultimately a vote - even if given with malice - is an expression of support for the candidate or party that receives it. If Ken Livingstone wins the election on the votes of people who can't stand him but hate Boris even more will he acknowledge that or will he claim that he is the chosen one of London?

Given that he has already burst into tears at the sight of "ordinary Londoners" calling out for their nasal-voiced saviour I think we already know the answer to that question.