55% doesn't seem right.

In the general hoopla about the LibDem-Con coalition, one thing has received little attention, though it has been picked up by a couple of people like Peter Hennessey.

This is that to make fixed term parliaments stickable, they want to change the law so that a government can't be brought down without a 55% majority against them.   But this is to confuse two things.   One is whether a government survives or not, and the other is whether to have a general election.   In principle they are separate.   The fact that David Cameron is PM now should not give him a lease until 2015, even if all the individual MPs do have that lease.    He needs to be able to be brought down in a confidence vote, possibly leading to another prime minister, either Tory or from another party.

For a confidence vote in the government, which means in practice in the Prime Minister, it should be 50% + 1 against to win.   Otherwise there is the ridiculous idea of a government which can't put any of its business through, but which can't be kicked out.

On the other hand, to stop the PM calling an election when he wants, you need a larger number than 55%.    Many PMs with a large majority could have got 55% of their MPs on their own.   Both of Tony Blair's first parliaments for example, and the second of Maggie's three victories.    So it doesn't really stop a dominant PM calling an election.   Even in this parliament, Tories plus Lib Dems have more than 55%, so could if they agreed force an election.

And this seems a nicely tuned number ... Con + LD is > 55% so can call an election anytime.  But Con >  45% so can't be defeated on a vote of confidence anytime.  So, theoretically, Cons could discard the coalition and form an invincible minority government.    Have the Lib Dems been sold a pup on this one?

In practice confidence votes to bring down a government are very rare - the last one in 1979 only months before the end of the parliament.    So it's unlikely this matters very much.  But my main point remains.  There needs to be the standard threshold for bringing down a government, but a much higher threshold than 55% to call an election.