So I’ve been thinking and I think the way to come out winning the last of the three debates, this Thursday, is to say something dramatic and new.

The first debate was obviously the game changer. People had the chance to see that there was a third real option, they liked it, as few as 1 in 10 people decided to switch to it, and the rest is history.

The second debate was always destined to be the quiet one - neither the first or last chance to see the leaders, supposedly focussed on foreign policy (though if you had tuned in hoping for some actual discussion of foreign policy issues you would have been fairly confused), and it was on the subscription channel (Sky). And indeed all it did was really confirm the impressions of the first one, just with Cameron behaving a bit more like Clegg, and Brown attacking Clegg instead of agreeing with him.

So what strategy for the third one? We’ve now had 3 hours of being able to watch these men and surely by now people have a pretty clear impression of what they think of them. Bar a few more tinkerings with the acting and the nuances of positioning which may tip things slightly in one direction or the other, we can’t really expect people to form a radically different view of the three men from their behaviour this time.

The only two ways it seems to me that the final debate could really change the position again, are these.

First, is the one we always knew about: that a leader could just make a brief and terrible mistake by saying something they really didn’t mean to. A few slipped up words could so easily have an impact out of all proportion to their actual significance, in giving the impression of that leader as incompetent, or really intending to do something that they have denied. Clearly all the participants will want to avoid this way of making their performance in the last debate memorable!

The only other way, it seems to me, is to say something new. Not a new major policy position - at this stage in the campaign that would simply give the impression that their manifesto isn’t a firm plan at all, but something flexible which they’re willing to change just to get a good headline. We have seen hints of this already with Cameron’s instant commitment on alleged cuts in last week’s debate (in response to Brown’s challenge), or Brown’s commitment on NHS funding to the RCN yesterday. At this stage, changing a policy position of any significance does considerably more harm than good.

But there may be other elements which a leader can throw out there, which are both new to the campaign, and actually important (suddenly deciding that you don’t like the phrasing of the commitment on marine biology on page 427 of a rival’s manifesto is not really going to cut it).

This could be a significant attack on one (or both) of your opponents. This needs to be something new and major, which may be a challenge at this stage in the campaign. But there may perhaps be new ways of putting this, particularly if it’s about their political positioning, rather than a policy. Labour is sure to try some version of this, along the lines of “Vote Clegg, Get Cameron”. Cameron will surely also try to sell a line which boils down to broadly “Vote Clegg, Get Brown”. Neither of these are very new - though if Nick is able to come up with an imaginative response to them which is not just the bathtub defence (”They’re both just the same, like squabbling children in the bath: vote Clegg and get Clegg”) then this may qualify as a killer line.

Or it could be something about their own positioning. Any of the three leaders proclaiming that in the event of a hung Parliament they’d be prepared to work with one of their rivals, but not the other one, would certainly qualify as this, but I doubt very much it’s going to happen.

But something about the way they would govern might do it.

Something which is a radical way of putting something which is already in their manifesto, perhaps implying they would go even further along a road already set out in principle in their manifestos, might do it.

All the parties have certainly left plenty enough room for more detail in their plans for sorting out the nation’s finances that they could outline more detailed plans here without being inconsistent with what they’ve already said.

And of course a good emergency would be ideal, allowing the leaders to give a distinctively different response to their rivals, and allowing them to start from a blank sheet of paper, relatively unconstrained by what they’ve said before.

It could be lots of things. It just has to be new, important, and not obviously inconsistent with anything they’ve already said.

This may not be particularly easy at this stage of the campaign. But it is surely the only way to use the last debate to give your popularity a real upwards boost, beyond the impression that the public already has of you.

I confess that I can’t really fathom what Labour’s campaign strategy has been this week. Ever since the first debate last week they seem to have been content to focus on taking David Cameron down, even if that benefits the Liberal Democrats rather than themselves. Is this just simple cynicism that even coming in third in the public vote, they could still have enough MPs to form a fairly stable government? Surely it can’t be – the British public’s lack of interest in their extraordinary voting system is great, but a party coming in third, with just 27% of the vote, and still forming the government just feels to me as if it would test the British public’s patience too far. I really can’t see a government formed on that basis lasting very long, and I can’t imagine that Messrs Brown and Mandelson would think so either.

No – to listen to Labour figures, all last week but especially this morning, condemning the attacks on Nick Clegg , it sounds as if they really aren’t worried because they think the Lib Dems will simply be so delighted to have the chance to go into government with them, that it is all a done deal. The only thing they need to do is to prevent the Conservatives having a proper majority, and they will get to stay in government.

This really is extraordinary. If this really is what they think, then they simply do not seem to have discerned the very basic insight that if Liberal Democrat MPs and members had wanted to support a Labour government, then they would have joined the Labour party. If you want, as Chris Huhne put it this morning, a cushy route to government, then you do not join the Liberal Democrats.

Liberal Democrat criticism of what this Labour government has been doing over the last thirteen years is not just grandstanding, it’s actually because we think they are wrong.

And Julian Glover at the Guardian makes this point extremely well.

But the extent of this delusion really does amaze me. The most startling appearance of it to me of all – and I promise I am not making this up – came right at the start of the campaign. My wife and I were in a hospital, in the run-up to the birth of our daughter, almost three weeks ago. We found ourselves discussing with a consultant two possible medical courses of action. He decided that the best way of illustrating the point of having to accept a less-than-ideal outcome, was to compare it to the Liberal Democrats’ prospects in the Election. As he explained, the Liberal Democrats might want to form a government, but they would be willing to support a government of another party as the best chance they were going to get.

In the context of discussing obstetric options, this was truly surreal. In the context of the election campaign, the idea that the Liberal Democrats – sympathetic though they may be to a one or two of the things that a tired and directionless Labour government has done – want to support it to remain there, is just wrong.

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…