Although there are fourteen candidates in the upcoming Eastleigh by-election - from the Beer, Baccy and Crumpet Party to more recognisable names (though no Greens) - this is primarily a contest between the two parties of the coalition government , with potential repercussions for the leaders of the three main parties.
Cameron probably has a lose-lose situation on his hands, due to the Conservative candidate, Maria Hutchings, who is generally considered to be on the right-wing of the party and a bit of a loose cannon - even allegedly being 'hidden' from hustings.[note Eastleigh Tory candidate Maria Hutchings: 'I am not being hidden']
If she wins, the victory will be heralded by Tory backbenchers as a sign that veering to the right is a clear route to electoral success, which may undermine Cameron's policy of attempting to modernise the party. This would be a shame, as I think Cameron is taking the country in the right general direction on some social issues (e.g. equal marriage), despite the protestations from some backbenchers. On the other hand, if the Tories come second to the Lib Dems, Cameron may come under fire from MPs who are unhappy with being in coalition. Anything less than second place would be disastrous for Cameron, and would probably restart talks of a leadership challenge.
The upside of choosing a right-wing Conservative candidate though is that UKIP seem to have backed off - at least in terms of not putting forward a high-profile candidate such as Nigel Farage - and their supporters might be willing to back a Tory - albeit grudgingly - over anyone else. The marginal nature of the seat might result in some tactical voting, along the lines of 'UKIP can't win here, vote Conservative to keep the pro-Europe Lib Dems out'.
Victory for the Liberal Democrats would no doubt boost Nick Clegg's standing within the party, which has taken a battering over the past year or so.[note Nick Clegg’s leadership woes] Betfair currently has odds of 4/11 on a Lib Dem victory, although PoliticalBetting.com shows mixed results depending on which opinion poll you read.[note Eastleigh: Now Survation for the Mail on Sunday has the Tories 4 percent ahead] Clegg has been making regular visits to the constituency, suggesting that the party still views him as an asset, rather than a liability.
Support from activists on the ground is strong for the Lib Dems - they know they need to win this. I know people who have travelled from Manchester[note 200 miles by car, over 4 hours by train] in order to join the campaign, and a Lib Dem member reported this week[note Politics Weekly podcast: Eastleigh byelection and mansion tax] that over 200 activists were on the streets, knocking on doors and delivering leaflets.[note 'The voters of Eastleigh want to beat, stuff and kick someone', Financial Times, February 23 - February 24 2013, p.2] This is the sort of campaign where the strong local presence of the Lib Dems should help propel them towards victory.
In terms of timing, the party's Spring conference two weeks after the by-election will provide an opportunity for Clegg to make a speech arguing that the current plan is working, or allow members to attack his leadership - depending on the result. However, unless the party's majority is wiped out and there is someone willing to wield the knife, I think he is still safe as leader until the next general election.
Miliband has the easiest ride here, as no one expects the Labour candidate to win - although this pessimism, together with the cold weather, may make it difficult to rally local activists. Any increase in Labour's share of the vote can easily be spun as a success story - all as a result of Ed's leadership of course.
Miliband's task is made even easier by the fact that Labour's vote fell significantly in Eastleigh at the last general election, so a return to the previous results will look like a triumph. Anyone who looks at the numbers[note 1997: 26.8%, 2001: 21.9%, 2005: 20.6%, 2010: 9.6% - source: Eastleigh (UK Parliamentary Constituency) on Wikipedia] will see that Miliband has just put the party back where it was before 2010, but the general public will only see the 'Labour vote share doubles' headline.
Overall, Cameron is likely to be disappointed, Clegg will probably be alright and Miliband will rejoice. Heads are unlikely to roll in any event.