With the election campaign fully under way, it’s high time to revisit the 60 constituencies that will play the biggest part in determining the result.

They are the seats Labour is hoping to take from the Conservatives, and it will be the advance Labour makes down this list that most shapes its success in advancing towards Downing Street.

I’ve divided the 60 into five categories, according to how much of a challenge I feel it will be for Labour to win. In each case I’ve listed the swing needed for the seat to change hands.

1. Easiest gains

Warwickshire North (0.1%)
Hendon (0.1%)
Cardiff North (0.2%)
Sherwood (0.2%)
Amber Valley (0.6%)
Broxtowe (0.4%)
Waveney (0.8%)
Wolverhampton South West (0.9%)
Carlisle (1%)
Morecambe & Lunesdale (1%)

These 10 seats should change hands pretty effortlessly and will be the foundation of any Labour victory. If the Tories hold on to any of them, it will be a massive upset for Ed Miliband and an enormous success for David Cameron.

2. Likely gains

Stockton South (0.3%)
Weaver Vale (1.1%)
Lincoln (1.2%)
Stroud (1.1%)
Bedford (1.5%)
Warrington South (1.4%)
Enfield North (1.9%)
Brentford & Isleworth (1.8%)
Northampton North (2.4%)
Nuneaton (2.3%)
Ipswich (2.2%)
Croydon Central (2.9%)
Harrow East (3.5%)

This group of 13 constituencies ought not to pose too much of a challenge for Labour – especially the four London seats – but there’s a slight chance the party could be tripped up in somewhere like Stockton South, where an opinion poll in November 2014 put the Tories ahead. Places like Nuneaton and Ipswich feel like they should fall to Labour fairly comfortably, but I can’t be as 100% confident about them as those in my first category.

3. Tricky but not impossible gains

Lancaster & Fleetwood (0.4%)
Plymouth Sutton & Devonport (1.3%)
Dewsbury (1.4%)
Pudsey (1.7%)
Hastings & Rye (2%)
Halesowen & Rowley Regis (2.3%)
Gloucester (2.4%)
Bury North (2.5%)
Kingswood (2.6%)
City of Chester (2.8%)
Keighley (3.1%)
Wirral West (3.1%)
Cannock Chase (3.5%)
Warwick & Leamington (3.6%)
Ealing Central & Acton (3.9%)
Norwich North (4.6%)
Dudley South (5.2%)

Here’s where things get a whole lot more unclear. On paper, most of these 17 seats should be gained by Labour with ease. But in places like Plymouth Sutton & Devonport and Cannock Chase I suspect the state of play on the ground is very different, with a strong Tory defence making things very hard to predict. By contrast somewhere like Wirral West is now looking like a pretty straightforward Labour gain, while Dudley South is well within the party’s grasp. As for Lancaster & Fleetwood, I’m wary of stating anything with conviction other than an improved showing by the Greens could, as in 2010, make it a very close contest indeed.

4. Unlikely gains

Thurrock (0.1%)
Brighton Kemptown (1.6%)
Hove (1.9%)
Erewash (2.6%)
Blackpool North & Cleveleys (2.7%)
Worcester (3%)
Loughborough (3.5%)
Swindon South (3.8%)
Stevenage (4%)
Carmathen West & Pembrokeshire South (4.2%)
Vale of Glamorgan (4.4%)

The likes of Brighton Kemptown and Hove are in this category thanks to the Greens looking like they’ll split the left-wing vote. Thurrock is by now probably a lost cause for Labour, thanks to the titanic scrap under way between the Tories and Ukip. There may be some gains here – if the student vote swings against the Conservatives in Loughborough, for example – but they will be the exception not the rule.

5. Hardest gains

Pendle (4%)
Elmet & Rothwell (4.1%)
Dumfriesshire, Clydesdale & Tweeddale (4.6%)
High Peak (4.6%)
Milton Keynes South (4.7%)
Cleethorpes (4.8%)
Rossendale & Darwen (4.8%)
Somerset North East (4.8%)
Great Yarmouth (5%)

I reckon pretty much all of these are now beyond Labour’s grasp. Time was the party fancied its chances in taking the Tories’ one remaining Scottish outpost, not to mention picking up the coastal seats like Cleethorpes and Great Yarmouth. But those days are long gone.

Adding up the figures, a clean sweep of categories one and two would give Labour 23 gains. If we say at least half of those in category three are likely to change hands as well, we get a total of around 30 gains: quite possibly not enough to offset the number of seats Labour is on course to lose to the SNP. But it might well leave the party almost neck-and-neck with the Conservatives, assuming both take a roughly equal number of seats from the Liberal Democrats.

The key for Labour is therefore how many additional gains they can pick up from categories three and four. If they can nose ahead in half a dozen more places, and maybe hold off a similar number of SNP challenges, they could well end up the largest party in parliament. But such an outcome feels, for the time being, more of a possibility than a probability. And therein lies the excitement of the next five weeks.