John Curtice: Saltire Scot or a Disappointed Democrat - which one are you?

IT all began in 1979 when Margaret Thatcher did particularly well in much of Essex. Her victory was said to rest on "Essex Man" - an escapee from the East End with a good skilled working class job who felt he was being taxed too much and was none too keen on London's racial diversity.

The stereotypical swing voter was born - the voter in whose hands victory or defeat supposedly rests.

Not to be outdone by the Iron Lady, two decades later Tony Blair conjured up an heir - Sierra Man (who after a model change became Mondeo Man). Much like Essex Man he too had moved up in the world, now owned his own house and lovingly polished his mid-range car on a Sunday afternoon. He too once feared that Labour would tax him too much, but returned to the fold after Tony Blair promised not to increase the basic rate of income tax.

In 2010 car drivers were again at the forefront of party strategists' minds. The outcome of the UK election was said to turn on "Motorway Man". Many a key marginal seat was located on or near the M1 and the M6. So the middle manager who relied on his car to do his job was supposedly set to put the Conservatives back into power.

However, Motorway Man did not quite deliver. And much as their imagery might help politicians gain media attention, the importance of such social stereotypes can easily be exaggerated. Elections are rarely won or lost because a party does well amongst a particular social group. For example, SNP support increased between 2003 and 2007 by almost equal amounts amongst those in well paid white-collar jobs as it did amongst those in routine blue-collar ones.

However, those who switched to the Nationalists in 2007 did have one distinguishing characteristic - they were often "Saltire Scots". They thought of themselves as Scottish rather than British, and were minded to want a stronger Scottish Parliament and perhaps even independence. They had not always backed the SNP before - but in 2007 were finally persuaded that the Nationalists might be able to do the country they loved some good.

Mr Salmond's fate in May will rest in part on whether he can persuade Saltire Scots that he has done enough for Scotland to deserve another chance. At the same time, however, both he and Iain Gray will need to set their sights on a new group too - the "Disappointed Democrats".

First generation graduates working in the public sector, Disappointed Democrats liked the Lib Dems because of their commitment to education. Now that faith has been shaken. What remains to be seen is to whom they will turn in May.

l John Curtice is professor of politics at Strathclyde University.