Much has been said about the challenges the Eastleigh by-election will pose for the two parties of government, and it’s true that either David Cameron or Nick Clegg will face awkward questions from their party after 28 February. But Ed Miliband has no cause for smugness just yet.

Having failed to even get 10 per cent of the vote in Eastleigh in 2010, there will be little pressure on the Labour candidate in the by-election, who will be announced today. Yet the seat is a test of the party's ability under Miliband to appeal to southern voters. John Denham, the MP for neighbouring constituency Southampton Itchen, is leading the Labour campaign, and highlights the election’s wider significance in Miliband's "One Nation Labour" project.

"The reason the Labour Party is fighting this seat seriously is not just because we want to get the best possible result here, but because, as a party that’s aiming to rebuild its base in the south, this is the sort of constituency, and the people who live here are the sort of people, that we want to represent.

"When we talk about being a One Nation Labour Party - south as well as north - we’re not just talking about the handful of places that might be target marginal constituencies at the next election but the whole of Southern England. If we can get that across really strongly, that in itself is a victory for us."

Predictably, Denham wasn't prepared to put a numerical figure on this "victory". Realistically, doubling the party’s share of the vote and breaking the 20 per cent mark would be a useful first step in Labour’s arduous task of regaining the trust of the south.

Such a result would rely largely on what Denham describes as "Labour-inclined [people] who voted Lib Dem in 2010 because they thought they were going to keep the Tories out and who are particularly bitter at the moment". Labour’s problem is that it's in their interests for such voters to bite their lip and support the Lib Dems again in 2015; the Conservatives are second in 38 of the Lib Dems’ 57 seats.

So while it’s easy to see the attraction for Labour in soft-pedaling in Eastleigh, it’s one that should be resisted. Light campaigning from the party might increase the chances of the Lib Dems holding onto the seat, but it would also be very risky: even without Nigel Farage as their candidate, Coral [2] make UKIP evens to beat Labour. If that happened, it would be hard to suggest that the party's one nation extended south of the M25.