BBC political correspondent Norman Smith looks at what is at stake for the political parties as they await the result of the Norwich North by-election.
This is a must-win by election for Team Cameron. Indeed failure to do so would risk bringing the Cameron juggernaut to a shuddering halt.
It could also be seized upon by Mr Cameron's critics, particularly those Tory grandees bruised by the leader's treatment of them over the expenses saga.
Not surprisingly therefore Mr Cameron's aides have been desperate to play down their prospects, insisting this is a "tight race".
However, given this is a former Tory seat, in the third term of a Labour government, with only a 5,000 majority to overturn, the Conservatives should sweep to victory.
It may not be a Glasgow East or a Crewe and Nantwich but Mr Cameron's people have to win with something to spare if they are to remain comfortably on course for a general election victory next year.
For Labour anything short of a complete meltdown will probably be bearable.
Privately, most Labour MPs have completely written off their chances with some fearful the party could even be pushed into third place.
While that would be grim, it is unlikely to provoke another threat to Mr Brown's leadership. Having bottled previous attempts to oust Gordon Brown, Labour dissidents are resigned to the fact that Mr Brown will lead them into the next election come what may.
Labour people also have a couple of handy excuses to explain away a bad result. First, they were handicapped by the Ian Gibson factor, with many local constituents still angry at the way he was treated by the Labour party.
And second, their candidate was stricken with a swine flu scare. Not the best backdrop for a by-election campaign.
THE LIBERAL DEMOCRATS
For a party that used to specialise in by-election upsets the Lib Dems under Nick Clegg have proved a bit of a damp squib in recent polls.
Nick Clegg, however, has managed to score some conspicuous political hits in recent months over the Gurkhas and Speaker Michael Martin.
He's also been quick on the media draw over Afghanistan and Trident and has been less damaged by the expenses scandal. So now he must hope his higher profile will, at last, start to pay dividends in terms of votes.
He also has a candidate who boasts impeccable local credentials. So for the Lib Dems, Norwich North has to be the moment to get that by-election bandwagon rolling again.
A win may be too much of a leap but they need to show significant progress otherwise questions will be asked about why Nick Clegg isn't connecting with the public.
In the middle of a recession green issues are not exactly flavour of the month. People are, perhaps understandably, more concerned about jobs.
So the Green Party are fighting at a difficult time and must fear a poor result would be cited as further evidence that environmental issues are sliding down the political agenda.
Their attention is also more focused on Norwich South where they claim to be breathing down the neck of Labour's Charles Clarke.
Nevertheless they may yet provide a shock upset if they prove more adept than the Lib Dems at hoovering up disgruntled former Ian Gibson voters.
They've also been involved in a bruising slanging match with the Lib Dems and so would probably regard overhauling them as a satisfying result.
UKIP, BNP and INDEPENDENTS
UKIP are still basking in the aftermath of their June euro election success in beating Labour.
However they start from a low base in Norwich North with only 2% of the vote at the last general election and no councillors. They'll hope to pick up some support from unhappy Labour votes but may suffer if euro sceptic voters opt to vote Tory instead.
Norwich North would not appear to be fertile territory for the BNP and a far cry from the North West where they have managed to gain some support. Indeed the BNP didn't put up a candidate here at the last election.
Among the others standing are the Libertarian Party, the Monster Raving Loony Party and Britain's former Ambassador to Uzbekistan who's standing as an anti-sleaze candidate.
Most will be grateful simply for an opportunity to air their views and gain some limited publicity.