Let's start with a bucket of cold water. For all the hype about tonight's Eastleigh byelection being the most important for 30 years, it's worth remembering that byelections themselves don't normally tell you very much about who's going to run the country after the next general election. "It cannot be stressed strongly enough that journalists, editiorial writers, party leaders and political scientists should be highly cautious about reading too much into individual [byelection] results," says Pippa Norris in British Byelections, one of the standard academic books on the subject. And Anthony Wells made a similar point on his UK Polling Report blog last night.



Anyway, let's assume the results are as the polls suggest – that the Liberal Democrats narrowly hold the seat over the Conservatives, UKIP do extremely well and Labour get squeezed down to fourth place. Journalists will write comment pieces concluding that the Lib Dems will do much better than the polls suggest as they'll still be able to get tactical support from Labour, that UKIP pose a serious threat to the Conservatives and that UKIP voters DON'T seem willing to vote tactically for the Tories and, for the Conservative leaning amongst them, that the poor Labour performance shows that Ed Miliband's "One Nation" mantra is just empty words. All of these conclusions are nonsense. Or at least, while some of them may very well be true, none of them will be things we can tell from Eastleigh. Firstly, if it votes in line with the polls Eastleigh doesn't really show the Lib Dems withstanding the national swing in seats they hold. In the final Ashcroft poll they were down 14 points on their general election score, which is pretty much what the polls show is happening to their national support. Secondly, I wouldn't conclude anything about tactical voting either Lab to LD or UKIP to Con – by-elections are very special cases, voters get an intense amount of literature and contact from the parties imploring them to vote tactically and send a message, and their vote won't change who governs so in many ways people are free to vote without consequence. The argument about Labour's one nation message is just point scoring – it is perfectly normal for a third party to be squeezed in a tight by-election and despite the exuberance of some Labour supporters at the start of the campaign it was bleeding bloody obvious from the beginning that Labour had no hope whatsoever in this seat.

Although Wells is exaggerating for effect, he's largely right.

And yet, as he goes on to point out, that doesn't mean byelections like Eastleigh aren't important. They are – not because they tell us anything particularly new about which party is up and which party is down, but because politicians think that they do. An election win is a solid event that confers authority, and after tonight either David Cameron or Nick Clegg (or conceivably both, but don't bet on it) is going to look like a loser. In a way that is hard to quantify, that will matter.

Eastleigh is interesting partly because it is only the fifth time since 1945 that the Lib Dems, or their predecessors the Liberals, have defended a parliamentary seat in a byelection. But the main reason why swarms of journalists have come to this Hampshire town to cover this campaign is that it is the first time since 2010 that the two coalition parties have gone head to head in a byelection. Cameron needs to win to show that he has a realistic chance of winning an outright majority in 2015. Clegg needs to win to show that, when the general election comes, the Lib Dems can avoid a wipeout. That's why it is billed as the most important contest of its kind for 30 years.

And there's another factor in play too. This byelection seems likely to mark another step in the rise of Ukip. In November last year in Corby, when Ukip got 14%, it was their best ever result in a byelection. Two weeks later it got 22% in Rotherham and tonight it could do even better. With not a single MP in the Commons, Ukip is still in many respects a fringe party. But, in byelections, it is establishing itself as one of the four main parties in English politics.

I'm at the Fleming Park leisure centre in Eastleigh, where the counting will start after the polls close at 10pm. As I write, the media consensus seems to be that the Lib Dems will win, and that Ukip are challenging the Tories for second place. But, of course, no one really knows. It should be an exciting night, and I'll be reporting on all the developments as they happen.

Here's the scene from where I'm sitting at the count.

Half an hour to go until the polls close.

Here is some background about the Eastleigh constituency.

The Eastleigh constituency

• The Eastleigh constituency Wikipedia page, with the results of all elections here since 1955.

• A ward by ward analysis of the political makeup of the constituency on the all that's left website.

• A Telegraph graphic with demographic and other information about Eastleigh

And here are the key election results from 2010.

Chris Huhne (Lib Dem) - 24,966 (46.5%)

Maria Hutchings (Con) - 21,102 (39.3%)

Leo Barraclough (Lab) - 5,153 (9.6%)

Ray Finch (Ukip) - 1,933 (3.6%)

Lib Dem majority - 3,864 (7.19%)

It's 10pm. The polls have closed. Some ballot boxes, containing postal votes, are already here and the verification process - checking that papers are valid - is now underway.

Unusually, there have been five opinion polls during the byelection campaign. Here are the results.



The opinion polls



A poll by Lord Ashcroft published on 8 February

Conservatives: 34%

Lib Dems: 31%

Labour: 19%

Ukip: 13%

Conservative lead: 3 points

A poll by Survation for the Mail on Sunday on 10 February

Lib Dems: 36%

Conservatives: 33%

Ukip: 16%

Labour: 13%

Lib Dem lead: 3 points

A Populus poll for the Times on 23 Feburary

Lib Dems: 33%

Conservatives: 28%

Ukip: 21%

Labour: 11%

Lib Dem lead: 5 points

A Survation poll for the Mail on Sunday on 24 February

Conservatives: 33%

Lib Dems: 29%

Ukip: 21%

Labour: 13%

Conservative lead: 4 points

A poll for Lord Ashcroft published on 26 February

Lib Dems: 33%

Conservatives: 28%

Ukip: 21%

Labour: 12%

There's a neat Guardian interactive graphic here which shows all five polls on a chart.

And here are two surveys of the polling published at the end of the campaign

• A final analysis by Survation



• An analysis by Anthony Wells at UK Polling Report

There are 14 candidates in all.

The candidates

Here are the main four candidates.

Mike Thornton - Lib Dem

Thornton is a classic solid, dependable local candidate. A business development manager, he's lived in the constituency for more than 15 years, he's served on the council for almost six years and he's campaigned as a local man championing local issues, not as an aspiring cabinet minister. He is not necessarily the most polished candidate on the ballot paper, but his opponents found it hard to find anything in his record to attack with much credibility. There are more details about him on his website.

Maria Hutchings - Conservative

"I am not a professional politician," Hutchings said on her leaflets. "I'm a straight-talking business woman and mother of four." The Tories tried to turn her outspokenness into an asset (read Boris Johnson on Monday), but a rash comment implying that local state schools were not good enough for her able son turned out to be the major gaffe of the campaign and there is no sign that her anti-Europe, anti-immigration stance has staunched the flow of votes to Ukip. There are more details about her on her website.

Diane James - Ukip



There was a time when Ukip candidates were noted for their flakiness and eccentricity, but James, a healthcare executive and a councillor in Surrey, has come over as mainstream and professional. The Guardian's John Harris said she was "a smart, apparently unflappable operator who you might easily mistake for an A-list Tory candidate" and in the Sunday Times Camilla Long said: "It is obvious that [James] should be the Conservative candidate, and ... Hutchings, a mother-of-four with unpalatable views on abortion and gay marriage, should be standing for UKIP." There are more details about James on her website.



John 0'Farrell - Labour

A comic writer and long-serving activist who wrote Things Can Only Get Better, a classic memoir about life as a Labour supporter, O'Farrell seems like a gentle soul, but, on the basis of what he wrote about his fleeting thoughts on first hearing about the IRA's attempt to kill Margaret Thatcher, the Tory tabloid press - and David Cameron - have depicted him in lurid terms as a terrorist sympathiser (which is a travesty, as I explained yesterday). Labour never had much chance here, and the smear campaign hasn't helped, but O'Farrell has livened things up with some wry tweets. There are more details on his website.

And here are the other candidates.

Colin Bex – Wessex Regionalists

David Bishop – Elvis Loves Pets party

Jim Duggan – The Peace party

Ray Hall – Beer, Baccy and Crumpet party

Howling "Laud" Hope – Monster Raving Loony William Hill party

Iain McClenann – National Health Action



Kevin Milburn – Christian Party "Proclaiming Christ's Lordship"



Daz Proctor – Trade Unionists and Socialists Against Cuts

Danny Stupple – Independent

Michael Walters – The English Democrats – "Putting England First!"

I spoke to all four main parties before the polls closed, and the Lib Dems and Ukip were certainly the happiest. They did not have any solid intelligence, but - for what it's worth - here's what they were saying.

Lib Dems: "It's going well," said a source. "Generally we are optimistic. We have had a good response." The party says it has had 600 activists in the constituency today - "coachloads of people coming in" - and other supporters have been making calls from home. The Lib Dems also think Ukip has done well.

Tories: The Tory camp sounded quite defensive. "It's between us and the Lib Dems, but it does look fairly close," said a party source.

Ukip: "We're a happy campaign," I was told. "We do not have to win to be successful." My source pointed out that Ukip started the campaign at 13% in the polls (see 10.19pm) and that it is now on course to do as well as it did in Rotherham, where it achieved its best result in an election to the Commons (21.7%). Ukip is also particularly flattered that the Conservatives have been putting out leaflets in Eastleigh using Ukip colours. "We are pretty confident of a great show, and that will send out a strong message that we are here to stay."

Labour: Labour believe that the Lib Dems will win. They think that the turnout will be high, that the momentum is with Ukip, but that the Labour vote is holding up.

All the boxes from polling stations are now at the count.

Nigel Farage, the Ukip leader, says he will be "very disappointed" if Ukip comes third, according to the Press Association.

LabourList are running a live blog about the Eastleigh result too. And msn news has got one on the go too, which you can read here.

Question Time is coming from Eastleigh tonight. I haven't heard very much of it, but PoliticsHome has filed the key lines. Here are what the participants have been saying about Eastleigh.

Angela Eagle, the shadow leader of the Commons, said the Tories would be "in meltdown" if they came third behind Ukip (as Nigel Farage seems to be predicting - see 11.16pm.) "If UKIP come second and the Conservatives come third, they've got a right to be very worried," she said.

Neil Hamilton, the former Tory minister who resigned over cash-for-question and who is now a senior figure in Ukip, said Eastleigh was now a three-way marginal. "My message is: get used to Ukip, because we will be providing the real choice [in 2015]," he said.

Claire Perry, the Conservative MP, said the byelection showed the need to link national arguments to local problems. "The lesson we should take away is, if we're trying to talk about the big important stuff like fixing Britain, we have to keep making it relevant," she said. "We have to keep getting out of Westminster."

Jeremy Browne, the Lib Dem Home Office minister, said the byelection showed that his party was resilient. "The big story for me is not that the Lib Dems are crumbling away; the big story for me is the resilience of the Lib Dems," he said.

If you were to apply the "who's looking glummest" approach to electoral forecasting, then the Tories are going to come third. There's a row of about a dozen of them, wearing rosettes and sitting on the elevated benches at the back of the hall, all looking as if they are attending a funeral. Party officials still insist that it is too early to know, but they don't look as if they are anticipating a triumph.

According to one party official, around 20 boxes have been counted already. There are 68 boxes in all to count.

The Lib Dems and Ukip are both saying that it's looking as if Ukip has come second, but they are not making firm predictions yet.

If the Lib Dems win, as we expect, and if Ukip come second, which now seems quite possible, the focus at Westminster will be on the Conservatives. Labour's Angela Eagle said the Tories would be in "meltdown" if they came third. (See 11.19pm.) But she was only echoing what David Davis said on Wednesday.

But if Labour do as badly as the polls suggest there may well be some sort of Labour party inquest too. Patrick Diamond, the former Number 10 adviser, has already kicked it off. In an article for the Guardian he says a poor result in Eastleigh would highlight the need for the party to do more to establish economic credibility.



Southern electors deserve an alternative to the coalition parties. The Conservatives and Lib Dems are perceived as economically prudent but harsh and unfair. Labour is viewed as caring but lacking the resolve to be a party of government. The voters deserve a governing party which ensures that even in tough times, economic efficiency and social justice go hand in hand.

We've got a firm prediction now. It's from the Labour camp, based on the what their counting agents have seen as they watched the ballot papers coming out of the boxes during the verification process. They think that the Lib Dems have won (just), that Ukip is in second place and that the Tories have come third.

That means Labour has come fourth. But the party thinks it has increased its share of the vote from the 9.6% it got in 2010.

The Conservative MP George Hollingbery is giving a radio interview now, about six feet from where I'm sitting. He has not conceded yet (or, if he has, I did not hear it), but he is in full "brave face on defeat" mode: the Lib Dems were always going to do well here because they hold so many council seats, governing parties only very rarely win seats in byelections (the last time was in 1982, when the Tories took a seat from a Labour MP who defected to the SDP and was rash enough to seek re-election), and "One Nation" Labour should be doing better etc.

We should be getting a turnout figure soon. The verification process (which involves counting the number of valid ballot papers) is over, the counters have had a break and the full counting process is about to start.

Richard Ward, the acting returning officer, has just announced the turnout. It was 52.8% (which is quite high for a byelection).

There are 80,775 people on the electoral role, according to the council. That means 42,649 people voted.

Tim Farron, the Lib Dem president, is here. He still won't claim victory - he says that the Lib Dems, the Tories and Ukip are all close, and that he's seen elections like this that are expected to go one way turn suddenly at the end - but it's obvious that he does think he's won. "If we can win this, after the week that we've had [ie, the Lord Rennard crisis], then flipping heck," he said. "And considering the circumstances of the byelection too."

He also said this was the most important byelection of his lifetime.

It's the most important byelection of my lifetime because, in every other one, if we had not won, there would have been another opportunity along the way.

Tim Farron may be being cautious, but Keith House, the Lib Dem agent here, is declaring victory. He's just told my colleague Steven Morris that the Lib Dems have won.

Here's some Twitter comment on the byelection.

From PoliticsHome's Mark Gettleson

Perhaps better pointer to 2015 than #Eastleigh: big Labour gain off Cons in Esther McVey's marginal Wirral West (Pensby & Thingwall ward)

From ITV's Chris Ship

Turnout in #Eastleigh was 53%. Postal votes was 66% (ie voted before #Rennard) Might be one factor for LibDem hold (if they do)

From the Daily Mail's James Chapman

Grant Shapps: 'Since WW2, there have been 485 byelections and gains have only been made by the main governing side on four occasions'

From the Spectator's James Forsyth



Labour coming 4th is a bad result for them—remember they came 2nd in 1994 by-elex. One Lib Dem jokes 'Labour; barely half-a-nation party'

From the BBC's Robin Brant

UKIP is the new protest party. Appear to be taking of all three established parties in #eastleigh. BUT Tories hardest hit almost certainly.

And here's Tom Watson, the Labour party's deputy chair, on the Eastleigh result on Twitter.

The single most important conclusion from tonight: David Cameron has never won a general election. If he can't win #Eastleigh he never will.

The Lib Dems think their majority is between 2,500 and 3,000, my colleague Steven Morris tells me.

Mike Thornton, the Lib Dem candidate, is just coming into the hall now.

In the past, when the Lib Dems won a byelection (which normally involved taking a seat off another party), there was a sense of exhilaration. Tonight they're happy, but the dominant mood is more one of relief.

Nigel Farage is saying Ukip had definitely come second.

Agents for all the candidates have been called to a table to adjudicate disputed ballots.

We should be getting the final result very soon.

Here's some more Twitter comment from the count.

From Greg Stone

Excluding WW2 coalition (Berwick, 1944), last Lib by election hold in govt was Banbury, June 22 1922. Also held Moray / Nairn day before. Correction to previous tweet - Libs were in National Govt at March 1934 by election hold in Combined Scottish Universities seat.

From ITV's Chris Ship

To bed while the Tories work out how to explain UKIP surge and Labour wonder why OneNation didn't go down too well in Hampshire

From the Telegraph's Michael Deacon

And here comes Nigel Farage, the man who might have won a by-election if only it had occurred to him to stand in it Why break the habit of a campaign? RT @rowenamason Mike Thornton, Diane James, John O'Farrell... No sign of Maria Hutchings

From the New Statesman's George Eaton

If Lib Dems benefit from incumbency factor in 2015 but their vote collapses in Lab-Con marginals, disastrous for the Tories.

Not long now. The Lib Dems are now saying their majority is around 1,700.

The candidates have been called to the front of the stage.

The results will be announced in five minutes.

We've had the results. Here are the key ones.

Mike Thornton, Lib Dems - 13,342

Diane James, Ukip - 11,571

Maria Hutchings, Conservative - 10,559

John O'Farrell, Labour - 4.088

Lib Dem hold - Majority 1,711

Mike Thornton, the new MP for Eastleigh, has just finished his acceptance speech. There was very little in it at all about national politics. Mostly it was a tribute to the work of the local council, although Thornton said the result did show those who have written off the Liberal Democrats that they are wrong.

In her speech, Diane James, the Ukip candidate, said the voters of Eastleigh had delivered a "humongous shock" to the political establishment. In the future, Ukip will never be excluded from political debate, she said.

Maria Hutchings, the Conservative candidate, sounded bitterly disappointed in her speech. She said that it had been a clean fight, and she paid tribute to the people of Eastleigh.

John O'Farrell, the Labour candidate, said he did warn Labour that he loses elections. He said that in future Labour would remain a presence in Eastleigh, and he started talking about the importance of politics. But at the back of the room people were already leaving. Everyone wants to get to bed.

Here's the full result.

EASTLEIGH

LD hold

Mike Thornton (LD) 13,342 (32.06%, -14.48%)

Diane James (UKIP) 11,571 (27.80%, +24.20%)

Maria Hutchings (C) 10,559 (25.37%, -13.96%)

John O'Farrell (Lab) 4,088 (9.82%, +0.22%)

Danny Stupple (Ind) 768 (1.85%, +1.56%)

Dr Iain Maclennan (NHA) 392 (0.94%)

Ray Hall (Beer) 235 (0.56%)

Kevin Milburn (Christian) 163 (0.39%)

Howling Laud Hope (Loony) 136 (0.33%)

Jim Duggan (Peace) 128 (0.31%)

David Bishop (Elvis) 72 (0.17%)

Michael Walters (Eng Dem) 70 (0.17%, -0.30%)

Daz Procter (TUSC) 62 (0.15%)

Colin Bex (Wessex Reg) 30 (0.07%)

LD maj 1,771 (4.26%)

19.34% swing LD to UKIP

Electorate 79,004; Turnout 41,616 (52.68%, -16.61%)

2010: LD maj 3,864 (7.20%) - Turnout 53,650 (69.28%)

Huhne (LD) 24,966 (46.53%); Hutchings (C) 21,102 (39.33%);

Barraclough (Lab) 5,153 (9.60%); Finch (UKIP) 1,933 (3.60%); Pewsey

(Eng Dem) 249 (0.46%); Stone (Ind) 154 (0.29%); Low (Nat Lib) 93

(0.17%)

The Lib Dems are used to sensational byelection victories. This is an important byelection victory for them, but it does not feel like a dramatic one. As I said earlier, the mood is more relief than anything else. At the end of the day they have only held a seat that has been Lib Dem since 1994. Their share of the vote is well down, from 46.5% in 2010 to 32% tonight, and their overall vote has fallen by more than 10,000. Admittedly, a defeat would have been a disaster. But avoiding disaster does not automatically constitute a triumph.

For the Tories, this probably is a disaster. In 2010 they had 39% of the vote here. Now it's down to 25%. Governing parties do lose support mid-term, but David Cameron cannot deploy that excuse because he doesn't govern with a Conservative majority. His hopes of winning the next general election outright depend on winning in seats like this. Over the next days and weeks he's going to find it hard persuading his party that this is now a realistic prospect.

Labour got 9.6% of the vote in 2010 and 9.8% tonight. Doing marginally better than in 2010 is not something to boast about, but, as the byelection inquests get going, Ed Miliband can at least take comfort from the fact that his authority has not been damaged as much as Cameron's.

The real winners are Ukip (if you can actually win an election by second). In November last year, when Ukip came second in Rotherham with 22% of the vote, it was their best ever result in an election to the House of Commons. They won with a 16-point swing. Tonight they got 28% of the vote on a 24-point swing. They have established themselves as the protest party of choice in the north of England and now in the south too.

I'll be blogging more on the consequences of this result, and on the reaction to it, tomorrow (or, technically, later today). But now I'm heading for bed.

Thanks for the comments.