Elections 2010: why the stakes are so high

Today's election will change the landscape of British politics. That much is certain. When it comes to the big headlines - and indeed a great many of the small details - there is genuine unpredictability.

The campaign has been mostly dull but nonetheless livelier than anticipated, defined above all by the increase in support for the Lib Dems which followed Nick Clegg's participation in the first TV leaders' debate. A major question here is whether the Lib Dems' increased support in opinion polls has been matched in the election itself. The party's degree of success - measured in both share of the popular vote and number of seats won - could be a decisive influence on what happens in a hung parliament.

This raises even bigger questions. Suppose the Lib Dems do as well as predicted by many polls, and Labour get as rotten a result as those same polls suggest. It's even possible (though I'm predicting it won't happen) the Lib Dems will take a higher share of the overall vote than Labour. This will quite possibly end the two-party hegemony of UK politics permanently, even if the 'third party' takes far fewer seats than the Big Two.

And that leads us on to another major possible consequence of this election: electoral reform. Clegg will make this central to any deal to be struck with either Tories or Labour in forming a coalition government (or offering support for a minority government). If some form of Proportional Representation (PR) is introduced, this will profoundly affect the future of national elections and is likely to alter the shape of Parliament.

Furthermore, if Clegg succeeds in making his party a credible choice for progressives and those who are broadly left-wing (or at least centre-left), it could erode Labour's place as the organised expression of social democracy in Britain. That would be a huge change indeed, and not necessarily one we'll see happen, but a big Lib Dem vote combined with collapse for Labour will make it far more feasible.

Another big issue tonight is the balance of seats between Tories and Labour. The former party is widely expected to take more seats, but there's no guarantee - and even if it does there's a vital difference between a slim margin and a big lead in seats taken. If the Tories open up a lead of 50 or more seats over Labour, David Cameron will be tempted to push (unconstitutionally) for demanding he gets to form the next government. The Murdoch press may well 'call' the election for the Tories.

A slim Tory win - or indeed Labour taking more seats - creates a very different scenario. Gordon Brown will probably feel able to pursue a deal with Nick Clegg: a Lib-Lab coalition of sorts will be on the cards. But this might still prove controversial if the Tories have decisively won the popular vote.

Any kind of hung parliament - whatever the precise balance - will be a fragile arrangement for this country's ruling elite. It is likely to lack the authority of a majority government as well as reflecting tensions in ruling class thinking more visibly than we're used to. This certainly matters at a time when business and politicans are determined to drive through severe cuts to public services and welfare.

For the left and the labour movement I suspect any permutation of a hung parliament will bring more benefits than losses. The weight of extra-parliamentary activity is increased when Parliament is weak, especially as politicians debate how far they can go in making working people pay for the crisis. A hung parliament will open up opportunities for the left and a number of campaigns and movements.

Join me from 11pm for live-blogging on Election Night.