Campaigning will get under way in earnest this week in the Oldham East and Saddleworth by-election, the first big test of public opinion since the creation of the coalition government.
Both Nick Clegg and Ed Miliband will pay their second visits to the constituency, which Labour won last year with a majority of just 103 over the Liberal Democrats.
David Cameron, who has faced accusations he secretly wants his coalition partners to win the contest on 13 January, is also due to visit this week.
In a sign of Liberal Democrat anxiety about their prospects, their former leader, Charles Kennedy, deputy leader, Simon Hughes, president, Tim Farron, and several ministers will also bang the drum for the party.
The by-election was called after the former minister, Phil Woolas, was stripped of the seat for smearing his Liberal Democrat opponent, Elwyn Watkins, during the general election.
The Liberal Democrats, whose poll rating has collapsed since their power-sharing deal with the Tories, fear being hit by the national backlash against the party.
Mr Watkins said Lib Dem ministers were "making a real difference to people's lives every day of the week."
Mr Miliband will say during a visit today: "The battle in Oldham and across the country is about Labour and this Conservative-led government.
"It may be a three-way fight for Oldham East and Saddleworth between Labour, the Liberal Democrats and the Tories. But it's only a two-way fight about the direction for the country."