15+ net Con losses and itâ€™s opposition for the Tories

For a country which according to Disraeli does not love coalitions, Britain seems to be doing its best to force its politicians into another one. That may well not happen, though not because anyone will gain a majority. None of the four likely largest parties post-election sounds keen on a formal pact where more than one of them has seats around the cabinet table. But whoever ends up on top needs a deal of some sort if heâ€™s to stand any chance of staying there for a decent period of time.

In 2010, there were various reasons why Clegg threw his lot in with the Tories. One was Gordon Brown and his acolytes proving far less co-operative than Cameron and crew; another was the political momentum â€“ Brownâ€™s government had all the aspects of a fag-end administration. But the clincher was simply the arithmetic. A deal with Labour never had the numbers to stack up and politics is the art of the possible. What would have been the point of lashing his party to a ship already holed beneath the waterline whatever the condition of captain and crew?

And arithmetic is likely to be the decisive factor this time too. For any administration to be viable, it probably needs to be able to call on the support of at least a couple dozen more MPs than its opponents can muster at the start of the parliament, to allow for rebels, defections and by-election losses. That means Cameron canâ€™t afford to fall below around 290 Conservative seats.

If the Tories do end up with 290, then we might expect something like Lab 253, SNP 50, LD 30, Plaid 3, UKIP 3, Green 1, Respect 1 and Northern Ireland much as now. Of those, the Conservatives can only look to the Lib Dems, UKIP and the various Unionists from Northern Ireland for potential support â€“ for the appropriate respective prices â€“ and even that would amount to only about 334 MPs: a majority of about 24 assuming six non-voting members (Sinn Fein and the Speaker).

A majority of 24 would be quite safe for a single-party government: by-elections are rarer now than the 1990s when John Majorâ€™s post-election majority of 21 was whittled away to into minority territory (no more than four seats have changed hands in any of the post-1997 parliaments), but this would be nothing like a single-party government. Indeed, it wouldnâ€™t even be a multi-party government.

Indeed, on those numbers, it might look at first glance as if Miliband would be in the stronger position: Lab+LD+SNP alone would give 333 and thereâ€™d be support from some minor parties too. However, that strength is also the weakness. Firstly, Labour would still be getting on for forty seats behind the Tories; secondly, whereas the Tories would only be absolutely reliant on the Lib Dems, Labour would need the consistent support of both Lib Dems and SNP. The notional constitutional advantage of being the sitting PM (i.e. that you can stay there and take the matter to Parliament) doesnâ€™t actually count for much. The reality is that if itâ€™s clear that more than half the Commons wants you out, thereâ€™s nothing you can do: a government with a life expectancy of days has no power. This is even more so now the FTPA has effectively split the processes of booting the old government out on the one hand, and forming the next one on the other.

Sill, with the SNP 100% committed to kicking the Tories out, the Lib Dems again assume the position of potential kingmakers as the key swing vote, even if theyâ€™re reduced to fourth place in the House. And just as they couldnâ€™t form a viable government with Labour in 2010, so any combined total less than about 320 means they couldnâ€™t give the Tories what Cameron would need either.

On that score, the key variable remains the Con/Lab marginal. The SNP may do better or worse but much the same number of Scottish MPs will line up behind Ed either way. Likewise, the great majority of the Lib Dem seats facing the SNP or Labour can already be written off and for Cameron, the remainder the Yellows hold only become of significant interest once the Tories are approaching an outright majority.

The failure of both UKIP and the Greens to break through in terms of seats therefore puts the emphasis back on the Con/Lab battle. Assuming the 30 or so Lib Dems I did earlier, that means that just 15 net losses for Cameron would see Ed in No 10.

David Herdson