If you believe the opinion polls, Ken Livingstone's chances of a third term as Mayor of London face the greatest threat to date in the shape of Boris Johnson. Under the rules of the Supplementary Vote system used for these elections (an oddly curtailed version of the Alternative Vote, although still fairer than a straight First-Past-the-Post contest), if the candidate with the most votes has less than 50% then all but the two leading contenders are knocked out and the ballot moves into a second and final round.
Here, voters for eliminated candidates who nominated a second preference for one or other remaining candidate get their votes transferred across to their respective totals. This means that - as the polls stand - people who choose to vote for the LIb Dem, the Green, Respect (SWP or Gallowayite), UKIP etc., will need to use their second cross for either Ken or Boris if they want a say in who gets to take up office in City Hall. Seems fairly clear?
Well now is perhaps time to reach for the anorak! The 2 round limit could distort the outcome somewhat, and even be decisive in the final outcome. Take a Green voter who also wants to register a protest, say about ID cards, and opts to give the second vote to the Lib Dems. It is entirely feasible that this voter might have ultimately preffered Ken over Boris, but the artificial "supplementary" vote which is not fully transferable could deprive them of making this count. So votes interchanging between lesser parties could, potentially, hand the job to Boris. Aaargh!