Today's Glenrothes by-election provides challenges for all the major party leaders.
Most significantly, can Gordon Brown - who was forced to break with convention and campaign as Prime Minister in the seat which neighbours his own - demonstrate that there has been a meaningful bounce in his ratings that will enable Labour to hold on to the seat?
Labour held the seat with a majority of 10,664 in 2005 and it still requires a swing of over 14 per cent for the SNP to snatch it. If Labour cannot hold on here today, it would not bode at all well for their chances of fighting a general election on the basis of being best-placed to handle the economic crisis.
But then the SNP did win previously safe-as-houses Glasgow East from Labour at the by-election earlier in the summer on a swing of over 20 per cent, and they start with an advantage in Glenrothes in that the party already holds the almost identical Fife Central seat in the Scottish Parliament.
It is vital for Alex Salmond that the SNP gains the seat if he is to show that momentum is still with his party north of the border.
The challenge for David Cameron is less signifcant in this contest, which is a clear Labour/SNP fight - and the electorate knows it.
Nonetheless, the party has the challenge of overtaking the Liberal Democrats to take third place - as it did in Glasgow East in July.
Nick Clegg, meanwhile, will be hoping that his party can avoid losing its deposit, as was the case in Glasgow East.
