The 2015 election will not see one party sweep to power by making gains right across the country. Success will be localised. A party might do incredibly well in one corner of the UK, but dreadfully badly in another. An advance in one region may be paralled by retreat in another.

This seems particularly true for Labour, who could end up in office thanks to the most inconsistent election-winning performance by a major political party in modern history.

I’ve drawn up a list of 12 seats that will serve as a guide to Labour’s fortunes, both at the polls and during the campaign.

Each of the 12 represents a particular kind of contest, a litmus test if you like, the result of which will provide a measure of Ed Miliband’s success – or failure.

I’ll do the same for the Conservatives and Liberal Democrats, both of whom are set for similarly inconsistent elections. But first, here is my pick of Labour’s decisive dozen.

1. Stockton South

Labour target no. 7

This is one of the most marginal Conservative seats in the country. James Wharton won Stockton South from Labour in 2010 with a majority of only 332. A swing of just 0.3% would deliver it into the hands of Labour. Yet an opinion poll published earlier in November suggested that far from trailing Labour, the Tories were actually one point ahead. If Ed Miliband can’t rely on taking constituencies like these, he has no hope of outperforming the Conservatives on polling day. Put simply, if Labour can’t win Stockton South, it will lose the election.

2. Carlisle

Labour target no. 15

Labour held Carlisle continuously from 1964 until 2010, when John Stevenson won the seat for the Tories. Stevenson’s majority is only 853 and a swing of 1% would send the constituency back into Labour’s column. It’s the type of seat Ed Miliband needs to win by a decent margin if he is serious about getting into Downing Street. A narrow victory would not bode well for the party’s chances of forming a government. The size of Labour’s majority will be worth watching here. Anything around 5,000 and the party ought to be heading back into power.

3. Brent Central

Labour target no. 23

A top Liberal Democrat target for Labour, and one the party should win very easily. Sitting MP Sarah Teather has already packed her bags, leaving her notional successor Ibrahim Taguri a majority of just 1,345 to defend. A swing of 1.5% would see the seat change hands. If Labour win big here, it will be a good sign of how the party is benefiting from a collapse in support for the Lib Dems and also how well it is doing in London. Conversely if Labour manages only a narrow victory, its long-expected (and much hoped for) sweep of up to a dozen Lib Dem seats might not be about to arrive.

4. Nuneaton

Labour target no. 37

Labour held this seat from 1992-2010, and before then from 1935 all the way through to 1983. It needs a 2.3% swing to win it back from the Tory MP Marcus Jones, and ought to be a fairly straightforward gain. If the party only just scrapes victory, however, it will suggest a very patchy performance across the country and imply Labour may not be doing well enough to overtake the Tories as the largest party in parliament.

5. Wirral West

Labour target no. 49

There is a prize Tory scalp for Labour in this seat: employment minister Esther McVey, who would be defeated on a swing of 3.1%. As someone who has been associated so publicly with the coalition government’s controversial shake-up of out-of-work benefits and allowances, McVey might be expected to be easy pickings for Labour. But she will be able to trumpet a lot of good statistics about the current level of employment in the UK, and if she survives at the 2015 election it will be a potent symbol of Labour’s failure to respond constructively to the government’s handling of the economy.

6. Swindon South

Labour target no. 55

The sort of seat that Labour absolutely has to take if it is serious about winning the election persuasively. A 3.8% swing would see the constituency change hands, but the omens are not good. Labour unexpectedly went backwards at the 2014 local elections and lost seats on Swindon council to the Tories. Robert Buckland won the constituency for the Conservatives in 2010.

7. Norwich North

Labour target no. 67

Chloe Smith won this seat from Labour on a huge 16.5% swing at a by-election in the dying days of Gordon Brown’s government. She held the seat in 2010 but with a reduced majority, and Labour now needs a swing of 4.6% to take it back. If it succeeds, the party could be heading for at least 300 MPs in parliament and even a slender majority.

8. Hornsey & Wood Green

Labour target no. 93

This will be a good benchmark against which to judge Labour’s success at taking further seats from the Liberal Democrats. The party is expecting to vacuum up at least half a dozen Lib Dem marginals comfortably. But how will it fare in seats like this, where it needs a 6.3% swing to unseat Lynne Featherstone? If Labour wins here, it could be looking at up to a dozen Lib Dem gains across the UK. If not, a valuable source of seats may prove to have run dry too soon.

9. Southampton Itchen

Conservative target no. 4

That rarest of things: a Labour seat in the south of England. John Denham held the seat in 2010 with a majority of just 192. He’s standing down in 2015, leaving the job of defending his wisp of lead to Rowenna Davis. The Tories need a swing of just 0.2% to take the seat. If they win, Ed Miliband will have failed at his most fundamental task: defending all the constituencies Labour won in 2010. Such an outcome would suggest the Tories could be in for a good election, quite possibly ending up once again the largest party in a hung parliament.

10. Great Grimsby

Conservative target no. 9 (and possible Ukip target)

Austin Mitchell is standing down here as Labour MP after 37 years. In 2010 he had a majority of just 714, and the seat is a top target for both the Tories and Ukip. Whether Mitchell’s successor Melanie Onn can see off both threats will be a key marker of how Labour is performing in its most marginal constituencies. Ukip’s candidate is Victoria Ayling, who represented the Tories in 2010. She has been tipped as Ukip’s best chance of taking a seat from Labour, and she’s been praised by none other than Mitchell as being a “very good candidate” – though he added he also thought she was “crackers”.

11. Dundee West

SNP target

This seat will provide a test of how well Labour has dealt with the challenge of the SNP. Jim McGovern has a majority of 7,278: the kind that in previous elections would have been described as safe. A swing to the SNP of 9.8% would be needed for him to lose. That sounds a lot, but if recent opinion polls are reflected at the election, Dundee West would fall to the SNP easily. Dundee voted Yes in the independent referendum. The constituency next door, Dundee East, is an SNP stronghold and seat of the party’s newly-elected deputy leader Stewart Hosie. Losing here would not necessarily mean Labour is having a dreadful election in Scotland, but it would suggest the party is on course to shed a handful of seats to the SNP, making the overall outcome more uncertain.

12. Glasgow South

SNP target

If the SNP triumphs here, Labour’s Scottish nightmare will have arrived. Tom Harris won a majority of 12,658 in 2010. The SNP needs a swing of 15.8% to unseat him. The constituency lies within another of the few parts of Scotland to vote Yes in the referendum. It also boasts some areas of strong, localised SNP support, such as the districts of Langside and Shawlands. If the SNP wins seats like this, Labour could see up to half of its Scottish MPs defeated and the loss of as many as 20 MPs: the kind of number that would rule out any chance of being the largest party in a hung parliament.