Leaders of the opposition used to participate in byelections only when they were highly confident that their party would win the seat, which isn't the case for Labour here. Yet both David Cameron and Ed Miliband feel they have no choice but to join Nick Clegg on the campaign trail. So in prospect is a short, sharp and potentially brutish contest that pits government against opposition, coalition minister versus coalition minister and leader against leader.
Byelections can have a special dynamic that means early polls often turn out to be deceptive. But the Ashcroft survey has encouraged the Conservative hunger to take back a seat they feel they ought never to have lost to the Lib Dems in the first place and which needs to be Tory again if they are to be in with a shout of outright victory at the next general election. A Conservative gain in Eastleigh would indeed be impressive, especially in the context of a double-dip recession that could turn into a triple-dipper.
The current Tory plan for the next general election is to target 40 seats, half of them Lib Dem held. Taking Eastleigh would make that seem more plausible as a path to victory. There is also something more visceral about the Tory enthusiasm for this contest: a basic urge to stick the boot into their coalition partners. In the opening shots of the struggle, the party chairman, Grant Shapps, has invited Eastleigh's voters to punish the Lib Dems on the grounds that Mr Huhne is a self-confessed liar now facing jail. Not all of the Tory chairman's colleagues are persuaded that this will be a profitable approach. Whatever his other sins, by most accounts Mr Huhne was an assiduous constituency MP.
There is also a tension among Tories about how hard to go at the Lib Dems generally. Some think the way to win in places such as Eastleigh is by suggesting to voters that the government would be more successful were the Conservatives not handcuffed to and held back by what one Tory minister of this mind calls "those pesky Lib Dems". Other Conservatives worry that attacking the Lib Dems in this way actually helps Nick Clegg's party by validating their claim to be the leash on the Conservatives.
They moved the writ very quickly – polling day is 28 February – in the hope of maximising that advantage. After so many humiliating results in other by-elections, a Lib Dem victory would be a huge fillip for Nick Clegg and help him to convince his party that their worst traumas are behind them. It might also persuade the Tories to think again about their plan to try to decapitate 20 Lib Dem MPs at the next general election and focus more on targeting Tory-Labour marginals.
And is it quite such a hopeless prospect for Labour as most people, including the bookies, are assuming? At the general elections of 1955 and 1966, Labour came within fewer than 1,000 votes of winning Eastleigh. Admittedly, the shape of the seat and its demographics have changed considerably since then, but more recent elections also suggest that Labour should not entirely write off its chances. The last time there was a byelection in the seat, in 1994, Labour came second, ahead of the Tories, with more than 27% of the vote. At the 1997 general election, Labour achieved a similar score.
It is then just about possible to envisage Labour winning the seat. I have heard Labour frontbenchers talk about "the Brighton scenario". Caroline Lucas won the Brighton Pavilion seat for the Greens with just over 31% of the vote because of the way in which the rest of the electorate split between the other parties.