In a return to her traditional song Polly Toynbee calls on everyone to vote Labour where they can, or tactically for the Lib Dems - but only when they are in contention with the Tories. To make her case credible she invokes a "very senior Lib Dem", who, she says,

expressed his anxiety to me that wobbly Labour voters may go Lib Dem in places they couldn't possibly win, accidentally gifting those seats to the Tories. If Cameron wins his overall majority, all chance of electoral reform is lost for years to come. Just as Labour needs Clegg to hold off Tories in their winnable seats, so the Lib Dems need the Labour vote to hold up and withstand a Tory assault.

In other words don't waste a vote on the Lib Dems unless you have to.  Well, Polly is wrong and so is that senior Lib Dem. For the extraordinarily open nature of this now three-horse race is that it has brought to life the dead electoral zones that usually cover the UK in political catalepsy.

Let me explain. Labour can't win. (Nor should we want it to, see George Monbiot. A Labour victory means a triumph for Mandelson and Brown, who must be hung.)

The Tories will get the most votes (unless something very brilliant happens, and the Lib Dems overtake them but this would only have been possible if the tabloids had backed Clegg).

The voting system means that the Lib Dems are likely to get less seats than Labour even if they get more votes.

In these circumstances the total number of votes the Lib Dems get matters. In fact it will be decisive. If they just get just a few more or less votes than Labour as the polls currently predict, they will lack the legitimacy to form an alliance with them. They would have to accept the voter's verdict and not overthrow a Cameron minority government. They could not be seen to keep Labour in power.

It will only be possible for the Lib Dems to form a governing alliance with Labour if the Lib Dems are the largest party in terms of overall votes. 

Then they could they insist that Mandelson and Brown resign and a new Labour leader agree to create a government with Clegg as Prime Minister (the scenario Will Hutton advocates).

The decisive issue here is the overall total of Lib Dem votes compared to Labour. This is much more important than whether or not the Tories gain a couple more or less seats. The Lib Dems have to beat Labour decisively in terms of votes to have the moral authority to form a centre-left alliance with them.

Why? Because there has to be change. A huge majority of the people are going to vote for this.

Therefore while Labour could support a Lib Dem led government as the change its supporters prefer, the Lib Dems could not back a Labour government to continue in office just because together they have more MPs.

I don't see how Labour could accept second place to the Lib Dems if it gets more votes as well as seats. Therefore every single Lib Dem vote counts especially vis a vis a Labour one. In exactly the situation Toynbee describes where the choice is either Labour or Lib Dem it is all the more important to vote Lib Dem for there to be any chance of 'keeping the Tories out'. Otherwise, where a a Green or a nationalist or an independent can win, support them - the Hang 'em strategy.

But there is one thing on which both Toynbee and Monbiot agree.

Both support the call for gathering in Westminster on Saturday after the election. Whatever the result of this election a new political period has opened that can lead to the UK becoming a constiututional democracy. This can't be left to the political class, of course, including the Lib Dems if they are forced into political deal-making. We need to take the demand for reform onto the streets now there is widespread public sentiment for a fair system where people can vote honestly and have an influence, without all the dreadful calculations of tactical advantage that is, frankly, corrupting. The colour, it seems, will be purple the colour of the franchise. It needs people not just politicians to make the change now.