From fitness classes and local gangs to the future of Parkhead fire station, it has not been the highest level of debate in Glasgow East, but the outcome will matter to at least three people at a very high level.

Tonight's result has the potential to provoke the downfall of Gordon Brown as Prime Minister, but it could also provide a significant check to the momentum of his two most dangerous rivals, north and south of the border. The voters of Glasgow East will have their say today and tomorrow they can watch the impact move swiftly into national politics. So if they vote for ...

A narrow Labour victory Bookies' and pundits are backing this option, despite more unpredictable variables than usual.

Any majority would get an audible sigh of Labour relief. This would stop the momentum of the Crewe and Nantwich by-election (a safe Labour seat lost in northern England) and then Henley (a humiliating lost deposit behind the Greens and BNP).

It would not solve Gordon Brown's leadership problems, but it might give him summer breathing space to plan how he clambers back up precipitously bad poll ratings.

The SNP would crow about a sharp cut in Labour's majority, and offer statistics about the wipeout of senior ministers, including Chancellor Alistair Darling, if the same swing were applied in the General Election expected in spring 2010.

It would also be quick to put pressure on Margaret Curran to stand down from her Holyrood seat, giving the SNP a second chance of defeating Labour in a by-election - though it should not hold its breath for that.

But even a narrow Labour victory would be a set-back, perhaps the most significant check to SNP momentum since it taking power at Holyrood. Alex Salmond's talk of an "earthquake" would look lame.

A narrow SNP victory Expect triumphant Nationalist air-punching. By-elections have long fuelled Nationalist fervour. The same swing at a General Election would unseat Gordon Brown from Kirkcaldy and Cowdenbeath.

But the attention would move swiftly to Labour and the future of Gordon Brown's leadership. It was signalled at the start of the by-election campaign by his critics at Westminster that defeat in Glasgow East would spark a revolt, or at least a delegation from party elders to tell him it is time to go.

If he fights to keep his job, a challenge would be hard to mount and could dominate politics for at least three months. There is no obvious replacement.

Although Westminster has started its recess, and Labour MPs next gather at the September party conference in Manchester, the Prime Minister still has to face a party pow-wow in Warwick today.

An SNP landslide Expect a headless chicken reaction from Labour, as MPs calculate the implications for their own chances of election survival. It could make for a cleaner break from the Brown leadership, while Alex Salmond's only problem would be constraining his party's expectations of a Labour wipeout at the 2010 election.

A Labour landslide A severe setback to Alex Salmond, not least for its surprise value. An endorsement of Gordon Brown's leadership, as he sets course to overturn the Tory Westminster poll lead, and the next Labour leader at Holyrood manoeuvres to oust the SNP minority government.

The battle for third place Both Nick Clegg and David Cameron need progress out of such by-elections - the LibDem leader because he still hasn't found much momentum and the Tory chief because he needs to show his appeal spreads north of the border.

If his candidate, Davena Rankin, cannot make headway from the Tory 7% share in 2005, it looks like his huge polling lead is more anti-Labour than pro-Tory. Neither man can afford to let one of the smaller parties defeat them.