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.