John Curtice: Coalition deals are here to stay

IT HAS already caused furious controversy. The SNP in particular is deeply upset. Meanwhile, this week their lordships in the upper chamber - potentially always something of an unpredictable bunch - are getting the opportunity to express their doubts.

What could possibly be causing this trouble? It is the proposed referendum on changing the electoral system for the House of Commons. The man behind the idea, Deputy Prime Minister Nick Clegg, wants the ballot to take place on 5 May next year, the day on which voters in Scotland are already due to elect a new Holyrood parliament. Indeed, there are elections in Wales, Northern Ireland and in most of England outside London too. Mr Clegg says combining the referendum with the various elections will save money. Those whose political futures will be on the line are fearful that their key election battle will be overshadowed by the Westminster-imposed ballot.

But apart from the row about the date, what is at stake in the referendum? Voters are being asked to choose between two systems for electing MPs. One is the "first-past-the-post" system currently in use. The other is the so-called "alternative vote".

Under this system, instead of just placing an X against the name of the candidate whom they would most like to see elected, voters are invited to place the candidates in order of preference, 1,2,3 etc. If no candidate wins 50 per cent of the vote, the votes cast for the candidate at the bottom of the poll are transferred to the remaining candidates in accordance with those voters' second and subsequent preferences. This process continues until someone gets past the 50 per cent mark, and thus has the expressed support of at least half of all those who have voted.

Who is likely to benefit from this alternative? As you might anticipate, it is Mr Clegg's Liberal Democrat Party. According to the polls, as of last May at least, many a Conservative and Labour (and indeed SNP) voter would give their second preference to Mr Clegg's party. As a result, some 20 or so Liberal Democrats who came a close second in May might well have been able to leapfrog into first place on the back of other parties' supporters second preferences.

More Liberal Democrats in the House of Commons would inevitably make it more difficult for both Conservative and Labour to win an overall majority.So switching to the alternative vote would seem to represent the death knell for Britain's tradition of "strong" alternating single-party government, whereby one party usually has an overall majority but where the government can always be turfed out at the next election.

That observation, however, immediately raises questions of its own. Did we not have an election under first-past-the-post in May? Did we not still end up with a hung parliament? Was not the colour of the next government decided behind closed doors? Is Britain not now governed by the very kind of government, a coalition, that first-past-the-post is meant to avoid? Perhaps we need to look once again at first-past-the-post and check it does indeed necessarily deliver what its advocates claim.

Maybe what happened last May was an accident. After all, had not the system come to be rigged against the Conservatives? In the average seat won by the Tories, 73,000 people were registered to vote; in the average Labour one, only 69,000. If that difference is eliminated - as Mr Clegg also hopes to do under a new set of rules for drawing up constituencies that could see the number of Scottish MPs cut to 52 - could we not expect normal service to be resumed?

In practice Mr Clegg's new rules will only partially reduce the disadvantage from which the Conservatives suffer. Even if they are eliminated - and that is far from certain - the differences between constituencies in the numbers of people registered to vote constitute only one of the reasons why the Conservatives find it particularly difficult to win a majority. More important, to focus on the Conservatives' particular difficulties is to miss the true significance of what happened last May.

Two key long-term changes have occurred in the way in which people vote that between them have made it more difficult for any party to win an overall majority. Consequently, even if voters opt to reject the alternative vote, we may have to get used to hung parliaments anyway.

The first change is that third-party MPs, be they Liberal Democrat, nationalist or whatever, have become much more common. Our first graph shows the number of MPs elected at each election since 1950 under some label other than Labour or Conservative. In the 1950s there were only a handful of such MPs, most of them Liberals. But from 1974 onwards the number of third-party MPs has grown. Scottish and Welsh nationalists have routinely been elected to the Commons. Politics in Northern Ireland have become divorced from those in the rest of the UK. And the Liberal Democrats have not just won more votes, but crucially have become much better at converting votes into seats.

As a result, at each of the last four elections, including 2010, at least 75 MPs have been elected under some banner other than Conservative or Labour.And, inevitably, the more third-party MPs there are, the greater the chances of a hung parliament.

Just as important is a crucial second development. In the event of a closely fought contest, first-past-the-post will only deliver an overall majority to whoever wins most votes nationwide if there are plenty of individual constituencies that are closely fought between Labour and the Tories. Otherwise, there may well be too few seats that change hands between those two parties to deliver a majority to whichever comes first across the country as a whole.

Our second graph shows the number of seats that were marginal between Labour and the Conservatives at each election since 1955. In the 50s and 60s there were about 160 such seats. But in the two 1974 elections there was a sharp fall, and the number has never fully recovered since. At present only just over 80 seats - or half as many as the 50s and 60s - can be regarded as marginal.This decline is a little noticed consequence of a familiar pattern - that Scotland and the north of England have gradually become predominantly Labour fiefdoms while Conservative strength has come to be concentrated in the Midlands and the south of England, leaving the party struggling to secure any representation north of the Border at all. Because Britain has divided politically into two halves, far fewer constituencies are now a microcosm of the country as a whole - and thus are places where both Labour and the Conservatives have a realistic chance of winning.

The combined effect of the rise in third-party MPs and decline in marginal seats has been dramatic. If Mr Cameron was to win a majority last May he needed not just the seven-point lead he actually secured, but as much as an 11-point lead. If Mr Brown was to keep his majority, he needed a three-point lead. Any outcome in between those two figures would have resulted in a hung parliament.

Those new rules for drawing up constituencies will reduce the lead the Tories need. But they will in turn increase Labour's target lead. Consequently, even if first-past-the-post is still in place in 2015, any reasonably narrow lead for either party is likely to lead to yet another hung parliament. The days of "strong" government may be over, irrespective of what voters decide in May.

• John Curtice is Professor of Politics, Strathclyde University