One of them, possibly both, is going to end up a loser

Given that the British electorate as a whole only gets to vote once every 4-5 years, it is perhaps inevitable that by-elections tend to wield a somewhat disproportionate influence on our political process.

Usually they follow a fairly predictable pattern. The public – or the media – identifies which of the two main opposition parties is most likely to give the governing party a good kicking, and the 'protest vote' does the rest.

But we are in unchartered waters now. There are two governing parties, only one main opposition party, and the new-ish leader of that party is something of an unknown quantity to most voters.

All of which is what makes Thursday's Oldham and Saddleworth by-election possibly the most fascinating such encounter since the Darlington contest of 1983 which, to Labour's ultimate detriment, saved the leadership of Michael Foot.

On the face of it, it ought to be plan sailing for Labour. In a few short months, the Con-Lib coalition has slashed public spending, put up VAT, and scrapped much of the regional aid budget that was helping former industrial towns like Oldham to find a new role.

Not only that, but the seat was won by Labour in the general election last May, before MP Phil Woolas was disqualified from Parliament for having lied about his Lib Dem opponent on his election literature.

One of the many imponderables in this contest is whether the fate of Mr Woolas will help or hinder Labour's cause here.

The fact that he was actually quite a popular local MP might appear to favour the party, but some voters may feel he was too readily hung out to dry by the Labour establishment, and punish the party accordingly.

Inevitably given its status as a three-way marginal, 'Old and Sad' is being viewed as something of a mini-referendum – not just on the Coalition, but on Labour leader Ed Miliband.

If Labour cannot even win by-elections in seats it already holds, serious questions will start to be asked as to whether it can win anywhere else under Mr Miliband's leadership.

But for Lib Dem leader Nick Clegg, too, a defeat on Thursday would be seen as a significant blow.

His party's candidate, Elwyn Watkins, may well feel the seat is his by moral and indefeasible right having been on the wrong end of Mr Woolas's misdemeanours, but politics doesn't work like that.

Undoubtedly the leader with the least to lose and the most to gain here is Prime Minister David Cameron.

For one, the Conservative candidate Kashif Ali started the contest in third place. For another, the Tories have been accused of soft-pedalling their campaign in a bid to help their Lib Dem partners.

Perhaps realising he is in with an outside chance of a stunning victory Mr Cameron is now doing his best to dispel that impression, becoming the first premier to campaign in a by-election for 13 years.

Yet ironically a Tory win, in a seat where the Lib Dems began as the main challengers, might end up being more destabilising for the Coalition than a Labour triumph.

Mr Clegg would then face more tough questions from his own activists and MPs as to what exactly the Lib Dems are getting out of the Coalition.

A Tory victory would simply reinforce the idea that it is the Lib Dems who are taking all the political pain while Mr Cameron reaps the political dividends.

But whoever wins on Thursday, at least one and possibly both of Mr Cameron's fellow party leaders are going to be waking up to some difficult headlines on Friday morning.

And for either or both of them, that could set the tone for a tricky political year ahead.