The upcoming by election in Eastleigh is a big one for the coalition parties. Not only is it a so far unique opportunity for them to compete with each other head to head in a parliamentary seat, it is also a seat that both would have reason to expect to win and recriminations can be expected when one of them doesn't.
I always make caveats about drawing conclusions from the overheated atmosphere of by elections, which are singular beasts in the political world, allowing parties to focus their entire national clout on 70,000 voters for three weeks. This campaign is no different, but conclusions will be drawn nonetheless, which may have quite far reaching impacts.
The Tories need to win seats like Eastleigh if they have hopes of winning a majority in 2015. If they fail, rumblings may get louder about the direction of the government and even the Cameron leadership - although I fully expect him to continue through the 2015 election. The Ashcroft polling initially puts them a nose ahead of the incumbents, with a 34% vote share. Shrewdly, they have picked a candidate who stood in 2010, so comes with some local profile and can also point towards her opposition to gay marriage, Europe and immigration. This makes her ideally placed to pick up tactical voters floating away from the 13% currently considering UKIP and she can also take comfort from Ashcroft's polling which shows Tory voters quite firm in their intentions.
Meanwhile, if the LibDems lose Eastleigh, a seat formerly occupied by an MP regarded locally as popular and a hard-working constituency member and in a seat where all 40 Eastleigh borough councillors within the constituency are Liberal Democrat (the other four seats are Tory, but not actually in the parliamentary constituency), then it will worry a number of Liberal Democrat MPs who have southern seats with similar majorities. Nothing concentrates the mind more than the rising fear that their political careers are about to end in failure rather earlier than they planned. After attacking the Tory candidate for not being local enough - a tried and tested tactic - they have sensibly picked a local councillor as their candidate, hoping to build on their continued electoral success at a local level. They will need to target the 19% Labour vote to boost their 31% vote share past the winning post, but Ashcroft reports that even the Liberal Democrat vote may be soft as they seem quite prepared to vote Conservative, even as Labour voters are prepared to back them to keep the Tories out - so we can expect the equally well worn tactic of squeezing Labour voters.
Historically, of course, you would have backed the Liberal Democrat by election machine to deliver a straightforward win, even starting 3 points behind, but with the Tories ahead and hungry for some good news, you wonder if the Liberals can field their usual army of supporters and whether they can outgun the Tories in spending terms. The Huhne effect is unclear - my suspicion is that voters won't seek to punish the Liberal Democrats for his behaviour in any significant way, but the loss of the incumbent personal vote might be damaging.
In the end, I think this is too close to call with any degree of reliability, but if I'm pushed, I suspect a Liberal Democrat hold on the 28th February. That is, however, three weeks away and in by election terms, that's a very long time.