The UK’s second-largest conurbation has a population of 2.5m. It has 27 parliamentary constituencies, over three-quarters of which are Labour. It is one of the party’s traditional bedrocks – which is why it has also become one of the party’s potential weak points.

For in Greater Manchester you will find plenty of Labour seats that have been taken too much for granted for too long. Among them is Heywood & Middleton, which the party almost lost to Ukip in a by-election at the start of October. Heywood was not a safe Labour seat – its majority was just above 5,000 – but it was nonetheless thought to be reliable. Dependable. Solid.

It proved to be none of those things, and those qualities must also now be discarded from many of Labour’s 22 Greater Manchester constituencies.

If Labour mounts a thoughtful, constructive defence of its seats in this region, there’s no reason it should lose any of them. It could end up even gaining a couple more. But a careless, presumptuous campaign might see this rickety northern fortress begin to crumble.

Here are the key electoral statistics:

Number of marginals: 11, or 40.7% of total

Number of ultra-marginals: 3, or 11.1%

Conservative targets: 6

Labour targets: 2

Liberal Democrat targets: 0

Liberal Democrats

Good news for Nick Clegg. In Greater Manchester, unlike almost every other area of the country, his party is not in danger of being wiped out at the election.

Of the Lib Dems’ three seats, at least one is unlikely to change hands: Hazel Grove. They might hang on to Cheadle as well. Both are Conservative targets, but would fall only on a swing far higher than that suggested by current opinion polls. The Lib Dems’ third and final seat will slip from their grasp, however. John Leech has no chance of holding on to Manchester Withington, which should be an easy gain for Labour.

Conservatives

Of the Tories’ two constituencies in Greater Manchester, one is ultra-safe: Graham Brady’s seat of Altrincham & Sale West. The other is not so safe.

David Nuttall in Bury North would lose his seat to Labour on a swing of 2.5%: the kind Ed Miliband needs for his party to be at least the largest in a hung parliament. It seems like the sort of seat Labour ought to take easily. But in another constituency needing a 2.5% swing, Gloucester, a recent poll by Lord Ashcroft suggested the Tories were currently ahead (albeit by one percentage point).

There is a top Tory target in Greater Manchester: Bolton West, where Labour’s Julie Hilling has a majority of just 92. Ukip won two councillors in Bolton in May’s local elections. If the party decides to target the seat, it could end up a three-way marginal. The other targets on the Tories’ list feel more implausible, with the likes of Stalybridge & Hyde (Labour majority: 2,744) and Bury South (3,292) a step too far – at least given the current opinion poll trends.

Labour

As mentioned above, Labour has one easy target here – Manchester Withington – and one less so  – Bury North. Everything else in Greater Manchester will be about defence: defence against the Tories, against Ukip, against taking absolutely anything for granted.

There are seats here that are open to a Ukip insurgency, although perhaps not one large enough to see constituencies change hands. Labour has eight seats that could be classed as marginal, including the recently-defended Heywood & Middleton. While the metropolitan seats will be safe from Ukip, the suburban areas could see Farage’s party playing havoc with swings and trends and taking great delight in disrupting Labour-Tory fights.

Ukip hasn’t yet stated it is targeting any seats in Greater Manchester, but I’m sure it will. What has previously been stout Labour territory is set to become just that little bit more precarious.