The Conservatives called it “Wobbly Thursday”; the Thursday before the 1987 election, when a poll – this was in the days when polls were few and far between, like rural buses – showed the Tory lead slipping to just four points.

Labour experienced its own Wobbly Thursday last week as panic spread through the party. Organisers in the “mainstays” – the seats the party held with narrow majorities in 2010 – began to sound the alarm. The "promise" – the number of people contacted by the party who have said they would vote Labour – was not holding up like it should.

The picture was worse in the party’s target seats. For most of the last year, when any two Labour staffers were gathered together, concerned has turned to the number of voters who didn’t know who they would vote for at the general election, who voted Labour in the council or European elections, but who said that the economy is growing. To make matters worse, canvassers were picking up Labour voters from 2010 who were now expressing doubts about Ed Miliband. “My expectation was that, thanks to the short campaign, those voters would be moving into our column,” one party strategist reports. Instead, they are moving away.

On Wobbly Thursday, the panic spread not just among the party’s staff in the field but also back to the upper echelons. Senior figures began talking to other MPs about the circumstances in which Miliband could stay on should the worst happen.

Word got back to the battlegrounds – “the numbers are bad at HQ too,” has become a constant refrain. Staff re-assignments only served to heighten the mood of worry. Parliamentary staffers who have been working in Labour's Brewer’s Green headquarters and around the country over the course of the short campaign are now being sent out to what are being described as "super marginals" – seats at the low end of the party’s target list. Places like Stockton South and Broxtowe have received extra staff, suggesting the party’s central data is projecting a tougher fight than expected by the polls – while alarmingly, seats like Pudsey, Ipswich and Northampton North are receiving no extra visits. Without those gains, even a combined Labour-SNP bloc won’t be sufficient to oust David Cameron.

One organiser sent me a text that summed up the overall message: “Dude, where’s my swing?”

“The only way I can explain our promise,” said another, “is if there is barely any swing at all.”

That would explain both the sudden outbreak of fear among the party’s field staff and the moves to shore up Miliband at the centre. It would explain why Labour campaigners in the party’s targets are fretful and its organisers in the mainstays are still nervy. But it could also be that Labour’s own information is faulty.

Remember that the party’s high command was blindsided by the defeat to George Galloway in Bradford West and its narrow victory over Ukip in Heywood & Middleton. Changes to Labour’s data collection technique since the last election may be causing an unnecessary outbreak of nerves.

In 2010, Labour ranked its own supporters on a sliding scale from one to five. “L5s” were, in the words of one campaign veteran, “to be reserved for people who have posters in every window, a garden stake on the lawn and a close relationship with the candidate”, all the way down to “L1s”, mostly non-voters “or the sort of people who say ‘Yes’ to everyone who comes to the door”.

The optimistic explanation for the decline in the party’s pledge is that the new system – which assigns a binary voting intention alongside a series of other questions – has led to over-enthusiastic data collection in the past that is now being exposed during the Get Out The Vote operation. Under the old system, another insider explains, “I would assign people to the candidate, to phone canvassers, and so on. Then I’d get rid of anyone below an L3 [before starting get out the vote operations].”

It may simply be that the decline in Labour’s vote is not a new phenomenon but one that has been masked by the new system – campaigners in one London target believe that over-enthusiastic canvassing earlier in the parliament means they are talking to voters who were never in the Labour column anyway.

Remember too, that Labour expects to lose, not just this election but almost every election. “This is the time that people wobble,” one senior staffer remarked recently, “I don’t.” Last-minute panics are what Labour has done at every election since 1992 "I always think we'll lose, and I've been wrong three times," was one reaction. Yes, Labour is worried. But we don't know if those worries are justified, and simply won't until Friday.