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Post-UK Debate Scenarios

I realize I’m going about this a bit backward — showing you the results from our new and improved UK forecasting model without having paused to fully explain it. But there’s a debate tonight at 8:30 GMT — the third and final one between the three party leaders — that could shake everything up anyway. If one of the parties were to gain or lose 4 points from their current position as a result of tonight’s event, for instance, it would make a huge difference in the composition of the next Parliament.

As a reminder, here is our current baseline scenario — how the models show the seat count turning out at a vote distribution of about Conservatives 34.5, LibDems 29.5, Labour 27:

But what if Gordon Brown were to follow on his ‘bigotgate’ gaffe yesterday with a poor performance tonight, costing Labour 4 more points and sending them down to 23 percent? They would be completely devastated. Assuming that Conservatives and LibDems roughly split the spoils, our model has them down to just 115 seats with a bare Conservative majority. This would be almost without precedent — about 70 percent of Labour’s current MPs would lose their seats.

On the other hand, if Labour were to regain its footing and add 4 points, they could still control a plurality of the Parliament:

(Labour would need to do a bit better than this — probably getting up to around 33-34 percent of the vote, to keep an outright majority.)

Suppose that the Tories’ David Cameron — who probably feels the least pressure heading into tonight — is the winner, giving Conservatives a 4-point poll bounce. Our model has them having a clear majority of Parliament in this case:

But if Cameron were to stumble and the Conservatives lost 4 points, we’d have a real mess on our hands, with everyone at least 70 seats away from a majority and the party having won the most votes (LibDems) controlling the fewest seats, and vice versa.

How far could a further surge by the Liberal Democrats carry them? Our revised version of the model is now hedging slightly more on the upside scenarios for Nick Clegg (even though it remains considerably more aggressive than alternatives). A 4-point swing to 33.5 of the vote would still only be good for second place in the seat count, and then only barely over Labour:

Instead, it would take more like a 6 or 7 point bounce to get LibDems to a plurality of the Parliament. (They’d need more like 10 points — to 39 or 40 percent of the vote overall — before they gained a majority.)

A 4-point collapse in LibDem support would be good news for the other two parties, of course — particularly Conservatives, who would be just on the brink of a majority.

So, there’s a lot riding on the performance tonight. With a four-point swing in either direction, Labour could end up with anywhere between 115 and 288 seats, Conservatives between 220 and 367, and LibDems between 73 and 184. That’s assuming, of course, that our model is getting this all about right, which it may not be — given the uncertainty inherent in forecasting this election, the ranges in practice may be even larger.

Nate Silver is the founder and editor in chief of FiveThirtyEight.