I don't believe that the Liberal Democrat surge will last (but here are three reasons why it might)
Well, I said 'be ready for froth' but I didn't expect quite such large dollops.
First we had some unnecessary confusion as ComRes ran (and published) a voting intention question to a sample that had all watched the debate, and then came back trying to sell us a 'remodelled, extrapolated' version, as if there could be any possibility of this being valid (you cannot possibly 'extrapolate' the opinions of non-viewers from viewers). Dismiss all that.
But then came the bombshell of our normal poll for The Sun showing the LibDems running a strong second, pushing back Labour to third. A real shocker, even allowing for margin-of-error. Is it really possible?
I stick to my froth theory for some of the observed effect, but maybe not for all. Consider this:
a) Experimental polls that ask people to imagine a genuine three-way race, with Conservatives, Labour and the LibDems all equal, and then ask respondents to place their votes, show a strong tendency for the main party vote to collapse to the LibDems. The fact is people do quite like the LibDems, they just think they can't win, and don't want to waste their vote.
2) Those experiments are sometimes supported by reality in the form of by-elections. When the LibDems manage to convince people they can really win, they often do, upsetting huge majorities. These new debates present the three leaders on equal terms, creating a little of that by-election effect.
3) Loyalty to the main political parties has sharply declined, especially after the expenses scandal, of which the LibDems were regarded (by the public) as relatively innocent. Many voters would love to give the main parties a bloody nose. The narrative is exciting, and creates its own momentum.
There are two more debates. A brilliant LibDem campaign could build on this unique moment and take us to the brink of a real convulsion. Even if they could sustain just half of the bounce, that would create havoc in the marginals.
And yet I still don't believe it. Of course I believe the numbers, I just don't believe they will last. There are two more weeks, and the big guns will start firing. They could blast the whole of that extraordinary bounce, and I expect they will. But the risks to the status quo cannot be wholly dismissed: as I said, the debates present the three leaders on equal terms, and can create something of the by-election effect. The LibDems could persuade the public this election is a special case. Momentum is a powerful thing (people are influenced by how they perceive others are thinking).
But I would need a lot more evidence before I believed we were really facing a systemic shock. I'm a great believer in mean reversion - more often than not, things tend back towards the usual.
One good thing to come out of this: no-one can remain complacent. The over-professionalisation of politics should be seen to carry risks; we do need a little threat to bring out some much-needed rawness from the main contenders. I look forward to the next two weeks being interesting. And then, probably, it will be back to normal.
Stephan Shakespeare is CEO of YouGov.