Deputy Prime Minister Nick Clegg will fire the starting gun in the Liberal Democrats' Eastleigh by-election campaign today with a promise to press for higher taxes on the wealthy to ease the burden on ''hard-working families''.
In the Commons, Lib Dem chief whip Alistair Carmichael will move the writ formally triggering the contest to elect a successor to disgraced former Cabinet minister Chris Huhne.
Polling will take place in just three weeks' time, on February 28, in a battle which will pitch the Lib Dems into a head-to-head fight with their Coalition partners, the Tories, who finished second in the 2010 general election.
Both parties are braced for a bruising contest, with Conservative Party chairman Grant Shapps signalling that they intend to focus on Huhne's past after he pleaded guilty in court to dodging punishment for a speeding offence.
''The residents of Eastleigh have for a long time been sold a lie by their Lib Dem MP. That's why they'll welcome the opportunity to express the way they feel about it very soon,'' he said.
''Conservatives will present the opportunity to start afresh with a hard-working local MP who gets on with looking after her constituents.''
Lib Dems insisted their decision, as the sitting party in the constituency, to opt for a short contest reflected the high state of preparedness of the local party, even though they have yet to choose a candidate.
''We are going to throw the kitchen sink at it. I'm pretty sure that the Conservatives will as well,'' one source said.
''It is going to be a tight contest, and generally in tight contests in two-way marginals you get some pretty robust exchanges.''
Mr Clegg is expected to travel to the Hampshire constituency to campaign as soon as he can clear the space in his diary. Officials would not be drawn on whether he would offer an apology for Huhne's conduct, but acknowledged that he would have to deal with the issue.
''Clearly the issue of Chris Huhne's resignation as an MP is going to be of relevance to the by-election,'' the source said.
Officially, the Lib Dems want to make jobs and taxes the key campaign issues - drawing clear battlelines with the Conservatives.
Mr Clegg will use a speech at the Institute for Government in London today to highlight the Lib Dems' commitment to a 1% annual ''mansion tax'' levied on properties worth more than £2 million - a proposal the Tories have consistently rejected.
Alternatively, he will say, the Lib Dems will press for the introduction of new council tax bands at the top end, again affecting properties worth more than £2 million.
''That we should ask a small number of very wealthy individuals to make a reasonable contribution, in order to provide desperately needed help for millions of ordinary people - nothing could do more to demonstrate a commitment to greater fairness in our tax system,'' he is expected to say.
''It's an open secret that our Conservative partners do not share our views on this. However, we will continue to make this argument, in this Coalition and beyond.
''Our approach is simple: taxes on mansions, tax cuts for millions. An approach to tax that puts payslips before palaces, if you like.''
The Lib Dems will select their candidate at a private hustings on Saturday. Officials insist they are confident of retaining a seat they have held since 1994.
Huhne had a majority of 3,684 at the last general election and the party has not lost a seat in a by-election for more than 50 years.