The rainbow coalition was doomed to fail; there is a strong dislike of electoral reform among Labour MPs (particularly in Scotland), so Labour could never have delivered the change that Brown offered to the Liberals, and Scottish Labour MPs refused to work in a government with the SNP.

I think there would also have been a wider, more divisive issue: with the SNP and Plaid Cymru in a coalition, with the stated aim of preserving funding for Scotland and Wales, the special deals involved ('pork barrel politics') would have fostered resentment in England, and perhaps done major damage to English voters' readiness to accept coalition government.