As the Lib-Dems agonise over which way to jump, a fascinating development this afternoon might make them think even harder about accepting David Cameron's offer.

Alex Salmond, the SNP leader, has urged the Lib-Dems to join in a progressive alliance with Labour, the SNP and Plaid Cymru to bring in electoral reform. This may seem strange, since on Friday Alex Salmond was telling the BBC's Election programme that he was not going to be part of any coalition. But what Salmond has now realised is that the numbers are just about there to bring in PR, if all those who want it get together.

Salmond is still not suggesting a formal coalition. But his intervention gives the lie to one of the most persuasive arguments for the Lib-Dems to join with the Conservative: the maths. Supporters of a Lib-Dem/Conservative deal argue that the maths don't add up for anything else. Labour and the Lib-Dems together would only muster 315 seats, still short of the magic 326 needed for a Commons majority.

Now that Salmond is offering SNP and Plaid Cymru support – even if it is only to bring in PR – that's another 9 MPs, bringing the numbers to 324. Caroline Lucas, the sole Green MP could be expected to join them too – 325. Then there are 3 SDLP MPs, or the 1 Alliance MP from Northern Ireland who may lend support and bingo – the once in a lifetime chance to change the electoral system.

As Nick Clegg is discovering today, his party members are far more reluctant to embrace a deal with the Conservatives than he and some of his colleagues. Interestingly, some Conservatives too are starting to voice their concern about the odd marriage of convenience. Alex Salmond's intervention could be critical at this stage – giving Clegg the reason he needs to reject the Tory offer, and to give his party the chance of electoral reform that has been the holy grail for so many years.

• More election comment from Cif at the polls