The Oldham East and Saddleworth by-election is the first big electoral test for the coalition government.
Labour is expecting to capitalise on anti-cuts sentiment, despite only narrowly holding onto the seat at the general election and losing its MP for knowingly making false statements during the campaign.
The Lib Dems, who came close to wining the seat, are desperate to succeed but may face a backlash from voters due to their cooperation in the deficit reduction plan.
The Tories are in a strong third place but David Cameron has been attacked in some quarters for conducting a 'soft' campaign.
Many commentators believe the prime minister would be pleased if the Lib Dems won the seat, as it would relive some of the pressure on his deputy and, therefore, the coalition.