In normal circumstances it takes millions of votes to bring about a change of government. However, in a marginal constituency, just a few hundred votes can bring about a change of MP. If the loser is a political ‘big beast’, the consequences can be far-reaching.

In 1997, Michael Portillo lost Enfield Southgate by 1,433 votes. No longer an MP, he was unable to stand in the leadership contest that year – an absence that may well have changed the outcome and thus the future development of the Conservative Party.

The preferred habitat of the big beast is a safe seat. Thus in most elections, they’re protected from the local accidents that might remove them from the national stage. In 2005 the Lib Dems attempted a ‘decapitation strategy’ aimed at five senior Conservatives – Theresa May, Oliver Letwin, Michael Howard, David Davis and Tim Collins. In the event, only Mr Collins succumbed – his head taken by Tim Farron (of whom, more later).

But what of 2015? A close election usually means that safe seats stay that way, thus allowing big beasts to rest easy. But these are interesting times – and on election night there are five seats that deserve special attention.

1. Paisley and Renfrewshire South

Though this is a finely-poised election for the UK as a whole, a landslide victory beckons for the SNP in Scotland. It seems certain to bury Danny Alexander of the Lib Dems, but the biggest potential scalp is his near namesake Douglas Alexander, the shadow foreign secretary and Labour campaign supremo.

According to UK General Election 2015 Paisley is number 40 on the list of SNP targets. Nevertheless, recent polls show it falling to the Nats. If the incumbent holds on, then Scottish Labour is left with some hope for the future, if, however, it’s a case of ‘were you up for Wee Dougie?’ then it’s game over. Poor Jim Murphy will likely survive in Renfrewshire East, but only as the captain who didn’t go down with his ship.

If the SNP then put Labour into government, Ed Miliband will have to find someone else for the FCO. But who? Step forward Alex Salmond: he’s always viewed the UK as a foreign country – and now he can be its Foreign Secretary.

2. Brighton Pavilion

With Natalie Bennett turning in media performances ranging from lacklustre to calamitous, Green Party members must be wondering why their former leader, Caroline Lucas, ever went away. The answer is because she’s got her priorities straight. Defending the Greens’ first and only seat is of critical importance to the party.

If Lucas loses, then the Green surge will have flopped. England’s ‘alternative left’ will conclude that Russell Brand was right about voting and retreat to the fringes. If, on the other hand, Lucas wins in Brighton (and Miliband in the country) she’ll be in a position to lead the radical opposition to a Labour Government – and turn the Greens into a serious electoral threat.

Last year, an Ashcroft constituency poll put Lucas just one point behind her Labour opponent. A follow-up poll put her ten points ahead – that, however, was at the height of the Green surge. Whether Bennett has put a dent in this advantage is uncertain, but either way there’s a lot riding on Britain’s most colourful constituency.

3. Thanet South

Brighton Pavilion is in some ways a ‘pre-decapitation’ – the closeness of the race forcing the Green Party to pull back its most precious asset. Much the same is true of Thanet South, where, of course, Nigel Farage is standing for UKIP.

Writing for the Telegraph, Iain Martin reflects on the importance of the battle:

“A difficult contest to win a seat of his own, in South Thanet, has turned into a traumatic struggle that the Tories are terming ‘Nigel’s last stand’.

“This threatens to upend the Ukip general election campaign, with Mr Farage so tied down against the Tories and Labour in Thanet that the national effort is having to be run from there. Resources are being poured into the seat. “If Mr Farage fails to win, there is the prospect of a bitter leadership race…”

It’s hard to say what a post-Farage UKIP would look like. Perhaps it would, as Martin argues, evolve along the populist lines envisaged by the Red UKIP tendency. Alternatively, it might descend into chaos and ultimate irrelevance – with Douglas Carswell soldiering-on as the George Galloway of the right.

If Farage’s gamble pays off, however, then he’ll get to strut his stuff in Westminster – a bear-pit that would suit him down to the ground. A Labour-led government would suit him even better and a Labour-SNP coalition best of all. While attempting to do to Labour in the North what the SNP is doing to Labour in Scotland, Farage could simultaneously snuggle up to disgruntled Conservatives on the green benches.

Without an equally big personality on the Tory front benches, there’d be a danger of Farage over-shadowing the official opposition. Consequently, a UKIP win in Thanet South would all but guarantee a Boris Johnson win in a Conservative leadership contest.

4. Sheffield Hallam

Picture the scene.

It’s about 3.00am on election night. The numbers are close, but it looks like the Conservatives will emerge as the largest party. They’ll need the Lib Dems and the DUP to stay in power, but David Cameron is confident that a deal can be done. He puts a call through to Nick Clegg and, as in 2010, pops the question:

“I’d love to, Dave,” comes the reply, “but you’ll have to ask my successor as party leader.”

The latest Ashcroft poll shows the Lib Dem leader two points behind his Labour opponent. If Clegg loses Sheffield Hallam, he loses the leadership – and David Cameron loses the Lib Dems. In such a scenario, it’s difficult to see the shell-shocked yellows making any deals until they elect a new leader – which would probably be the aforementioned, left-leaning, Tim Farron.

Should this be the course of events, Mr Farron’s path to power would be marked with not one, but two, decapitations.

5. ‘Constituency X’

While the SNP have ruled out a deal with the Conservatives, both the Lib Dems and the DUP have said that they will talk first with the party that has the largest number of seats. This could be a close run thing, with only a handful of constituencies determining pole position. Indeed, it could come down to a single seat – most likely one of the Con-Lab marginals, each of which makes a difference of two to the comparative seat totals.

In such a scenario, the big beast in question wouldn’t be the Conservative candidate for ‘X’, but David Cameron himself.

Though I’d like to be a bit more specific about where Constituency X might be, in this multi-dimensional election it’s hard to narrow down the possibilities. However, with some key seats not declaring until Friday lunchtime, it could be a very long night!