Leading article: The Liberal Democrats must now argue their case for electoral reform

Electoral reform was a central pledge of the Liberal Democrats' election manifesto.

And when the party was in talks with the Conservatives, it was David Cameron's agreement to advance legislation for a referendum on the Alternative Vote system that arguably clinched the deal. It allowed what might otherwise have been a loose concordat on parliamentary co-operation to become a formal coalition. It also trumped Labour's strongest coalition card in appealing to the Liberal Democrats: Gordon Brown's late conversion to AV.

As the weeks passed, however, the Government's silence on a timetable for electoral reform was starting to cause unease among Liberal Democrats. It seems that their agony – on this score, at least – will come to an end early next week when Nick Clegg announces that a referendum on AV will be held on 5 May next year, to coincide with local elections in England and assembly elections in Scotland and Wales.

So, the Liberal Democrats will finally have the opportunity to submit their ambition to the judgement of the people, and perhaps change the make-up of Parliament forever. They can hardly ask for fairer than this. Yet several potential obstacles stand in their way. The first, the complaints voiced yesterday by the Scots and the Welsh about holding the referendum on the same day as their own elections, can be discounted. It makes perfect sense, logistically and in cost terms, for the voting to take place on the same day.

The second concerns the referendum itself. The introduction of AV would be a relatively modest reform, but it will not be the only subject of the referendum. There will also be a question about redrawing the constituencies to give them more equal numbers of voters, which would probably also mean a smaller number of MPs. While the two issues are separate, their appearance on the same ballot paper could confuse the arguments. Many sitting MPs, with an eye to their future, might be so opposed to the constituency changes that they focused on this rather than on AV.

The third is that David Cameron will be campaigning to retain the first-past-the-post system. This system has an emotional appeal to many voters – as the only system they have known – and the practical draw that it (usually) produces a clear result. Mr Cameron can be counted upon to present these arguments in his usual cogent way, and with the additional authority that comes from being Prime Minister. Mr Clegg and his party will have to mobilise all their resources to counter them.

What is more, with the two leaders ranged on opposite sides, there are obvious risks for the coalition. And the risks are far greater for the Liberal Democrats than for the Conservatives. Defeat for AV would mean not only loss of face for the smaller party, but the demise of an issue that has helped define it. Their political platform would then look very threadbare well before the next election.

But that is to jump ahead. In the more immediate future, we look forward to an invigorating constitutional debate of the sort we have too rarely in this country. AV, from the point of view of this newspaper, leaves much to be desired; it is not proportional representation (PR). As a step towards modernising our democracy, however, it would be a shift in the right direction.