South Australia was Labor’s forgotten triumph of the 2007 election, replicating on a smaller and less spectacular scale the decisive tectonic shift in Queensland.

The statewide two-party swing to Labor of 6.8% was only slightly below Queensland’s 7.5%, which was borne out in the proportion of seat gains: three out of 11 in South Australia, nine out of 29 in Queensland.

Labor’s resurgence put an end to a slump that dated to 1987, the last time it had won a majority of the South Australian two-party vote, and 1990, when it last won a majority of seats.

Before that the state had been a source of strength for Labor in the postwar era, notwithstanding that a dubious electoral boundaries regime kept it out of office for much of that time at state level.

This was partly because the state party branch was spared the worst of the 1954-55 split, but also because of the large blue-collar workforce required to service an economy based largely on manufacturing and industry.

The difficulties experienced by these sectors meant the state was hit hard by the economic upheavals of the 1980s, which together with the damage done to Labor by the 1991 State Bank collapse led to a fundamental electoral shift in the Liberals’ favour.

At federal level this was manifested in a series of grim federal election results that reduced Labor to two seats out of 12 in 1996, to which only one seat was added in later terms of the Howard government.

With one seat having been abolished in 2004, Labor’s doubling of its representation at the 2007 election gave it a bare majority of six seats out of 11, and left the Liberals without a safe seat in Adelaide.

The two Liberal hold-outs in the city were Christopher Pyne’s seat of Sturt and Andrew Southcott’s seat of Boothby, which cover the traditional party strongholds of the east and inner south.

In a tale that will become increasingly familiar as this series proceeds, speculation about the coming election was long focused on the Liberals’ chances of retaining these existing seats, but such talk faded as the new year began and disappeared with Labor’s poll collapse over the past two months.

Labor’s main strength in South Australia lies in the coastal plain north of the city centre, which makes a safe Labor seat of Port Adelaide and marginals of four others that are leavened with more conservative areas beyond.

The electorate of Adelaide covers inner suburbs north and south of the city, which are respectively strong and weak for Labor, and the growing inner-city apartment population in between, which has proved highly volatile in its electoral habits of late.

In a rare sighting of the “doctors’ wives” effect, Labor’s Kate Ellis bucked the trend of the 2004 election to win Adelaide from Liberal incumbent Trish Worth, and she emerged from the 2007 election with what seemed like a secure 8.5% margin.

However, the Liberals are talking of internal polling showing them “closing the gap”, after staggering swings were recorded in the electorate at the March state election (at which Education Minister Jane Lomax-Smith lost the state seat of Adelaide with a swing of 14.4%).

To the west of Adelaide is coastal Hindmarsh, which combines Labor-voting inner city areas with prosperous and conservative Glenelg in the south. Labor’s Steve Georganas won by the narrowest of margins when popular Liberal member Chris Gallus retired in 2004, before picking up a relatively modest swing in 2007.

North-east of the city centre is Makin, home to newer suburbs in the hills along with the eastern part of Salisbury on the plain. Makin is the only seat in the state that has form as a bellwether, being held by Labor from its creation in 1984 until 1996, Liberal through the Howard years and Labor’s Tony Zappia since 2007.

Further north is Wakefield, which offers even starker contrasts: deep red Elizabeth in the south, rapidly growing Gawler just past the city’s northern limits (where change is favouring Labor, if the state election is anything to go by) and conservative rural and wine-growing areas beyond.

Wakefield was a safe Liberal country seat until it absorbed Elizabeth at the redistribution before the 2004 election. Liberal candidate David Fawcett unexpectedly retained it for the Liberals on that occasion, but his narrow margin was eliminated by Labor’s Nick Champion in 2007 (Fawcett now stands poised to enter the Senate).

The only seat in Adelaide that conforms neatly with the mortgage belt marginal seat stereotype is Kingston, covering the city’s outer southern coastal suburbs. Labor’s Amanda Rishworth recovered this seat for Labor in 2007 after it was lost in 2004, interest rates having had a lot to do with it on each occasion.

The diversity that characterises the other marginals is highly significant, as it leaves their members as susceptible to rebellions in party heartlands as to the normally more decisive ebb and flow of the mortgage-payer vote.

This is where the mining tax could cause problems for Labor, as many blue-collar workers perceive a connection between the mining boom and the industrial and manufacturing sectors that employ them.

While South Australia is rarely given a guernsey as a “mining state”, BHP Billiton’s massive Olympic Dam project single-handedly allows the industry to punch above its weight, as it is associated in the public mind with the state shaking off its “rust belt” reputation from the 1990s.

Uncomfortably for Labor, BHP Billiton says the tax will jeopardise a $20 billion expansion to the project, which is presently under consideration, a process that will certainly not be completed before the election.

Premier Mike Rann captured attention last week when he claimed any decision to stall the project would cost Labor four or even five seats.

For all that, the Liberals have big hurdles to clear if South Australia is to produce any of the seats it needs to overhaul Labor’s majority.

The problem is a lack of low-hanging fruit — even the most marginal of Labor’s six seats, Kingston, sits on an imposing margin of 4.4%.

Furthermore, the March state election suggests Labor has a trump card in the form of a ruthlessly efficient marginal seat campaign machine, which helped Rann hang on to office with just 37.5% of the primary and 48.4% of the two-party vote.

The only seats in the state that swung to Labor were the two most marginal, Light and Mawson (respectively in Wakefield and Kingston federally), and the critical eastern suburbs seats of Hartley and Newland likewise held firm against a torrid tide. Elsewhere, Labor suffered double-digit swings nearly everywhere they could afford to.

Federal Labor will be hoping to achieve similar successes in working-class areas with a campaign to focus minds on industrial relations, thereby shoring up valuable support in Makin and Wakefield in particular.

Beyond Adelaide, the state’s three non-metropolitan seats are of limited electoral interest, notwithstanding the vague threat the Democrats and now the Greens have posed in Mayo, where Jamie Briggs struggled over the line in the September 2008 by-election that followed Alexander Downer’s resignation.

That leaves Barker in the state’s east, which covers rural territory that has never been of interest to Labor, and the outback electorate of Grey, which  has transformed over the past two decades from safe Labor to safe Liberal — testament to the decline of the “iron triangle” cities of Whyalla, Port August and Port Pirie, and reflecting the experience of Kalgoorlie west of the border.