One of the things that I’ve been interested to note over the last few days, is that once the post-debate Lib Dem surge happened, their share of the vote pretty much stabilised at about 28-32%. While it’s obviously mathematically possible that this is a different 28-32% of people each day, it’s surely more likely that there is a relatively stable group of people who – at least this week – would vote Lib Dem.

I must confess I was a bit surprised by this – as I had kind of assumed that the rise itself would “liberate” a few more people to feel that a Lib Dem vote was no longer wasted, and so they too would give it a try. This is surely implicit in the whole “wasted vote” argument – if when the Lib Dems actually are doing really well, this doesn’t cause more people to take them seriously, this implies that their share of the vote has in fact never been historically depressed by the “wasted vote” argument, as campaigners of all parties believed.

So I was interested this afternoon to see the latest BBC’s “poll of polls” tracker page. It shows that I’ve got it slightly wrong.  What this shows is that the debate immediately triggered a sharp rise in the Lib Dem vote, from perhaps 21 to 29%. But it also shows that over the four days since hitting 29%, it has continued to meander slowly upwards, now to 31%.  This implies that there is indeed some kind of snowball effect – albeit for the moment limited. If there were no further debate tonight, then I’d expect this to continue to rise over the next day or two – to perhaps 32% - since the dates the BBC uses are those of poll publication, not of the fieldwork. After that, who knows? There surely does have to be quite quickly a correction from the fairly ridiculous hyperbole about Nick Clegg that has been going around. I’m a longstanding very firm Nick supporter – and his performance last week is almost exactly the reason that I and many others backed him so vigorously for the leadership in 2007 – but even I do not believe that he somehow combines the leadership skills of Winston Churchill and Julius Caesar, together with the forensic and brilliant debating abilities of Socrates and Charles James Fox.

But of course there is another debate tonight, which has the potential to change the picture all over again. It could set the Lib Dems off on another major leap upwards. Nick could say the wrong thing just once and that push us all the way back down to 20% again, or lower. The lightweight one or the overly-heavyweight one could say something which becomes the story, pushing their own share of the vote significantly either up or down. One thing we do know is that the media look for a story from the debate, which then reinforces itself as the story, only very loosely based on what actually happened (yes I think Nick did best last week, but it was not the kind of Lib Dem triumph and Labour and Tory disaster which you would think it was if all your information on it had come from any media source since, say, Saturday).

My guess is that tonight won’t do any of these things. We have had one General Election Prime Ministerial debate in this country, and it completely changed the game. I think it would be very rash to assume that they will all do that. I think the fact that that was the first real chance for voters to see all three Prime Ministerial candidates together was what really mattered, and particularly to get a proper look at the third party for the first time. I expect the impact of the remaining two debates (and tonight especially) to be possibly significant, but much more limited.

But of course we will see…