"Yes, I've always voted Labour in the past, but you know …" he said with a shrug as the door was slowly closed on me. Put him down as an L3 (half-hearted Labour supporter) who needs more persuading, I thought to myself. There were plenty more doors to knock on and I had to move on.

But what struck me was how many others like him I encountered that day. It's not unusual to meet voters who like campaigners to work for their support. Sometimes it's just to let off steam; sometimes they have specific concerns; others want to question party policy. But there was a nagging feeling that in an area that should be solidly Labour, the enthusiasm was tepid rather than full-throated.

If this were just anecdotal evidence then the party would have little to fear. It is now just over six months since Ed Miliband became Labour leader. The party's polling lead over the Conservatives is between 6 and 9% and if an election were to be held tomorrow, Labour would theoretically win by a landslide.

A poll just after the big TUC march found that just 24% of the public backed government plans to reduce the deficit. Only 3% thought there should be more cuts. In contrast, 64% were opposed to government policy on cuts.

But paradoxically, both sets of figures underline how tepid Labour's own support is.

Consider this. Public opinion has consistently moved against the government's arguments on the fairness ("we're all in this together") and the speed of cuts since last year.

There's also this stark trend on the public's perception of how the government is handling the economy.

And it's notable how quickly the public has started to mistrust Conservative plans for the NHS.

All this has taken place despite Labour's muddled response. The party jumped from 29% to around 45% in voting intentions even though most people know little about Miliband.

Conventional wisdom in Westminster dictates that Labour can become electable again only if it accepts Osborne's deep cuts. But this isn't just economically illiterate, it also deliberately ignores the shape of public opinion.

The above figures emphasise how fundamentally mistrusted Conservatives still are by voters. Just before the election, internal Tory polling found that while 75% of voters believed it was time for a change from Labour, only 34% believed that change should be the Conservatives. In the end, their share of the vote wasn't much higher. David Cameron was convinced voters would warm to the Conservatives after seeing them in power. In fact the opposite has happened: the public's traditional mistrust has been swiftly confirmed.

But this also exposes Labour's weakness. While a larger percentage of voters are sympathetic to Labour, they aren't enthusiastic enough about the party to support it at elections. In fact, they are the party's achilles heel.

To his credit, Miliband started talking about Labour's "lost voters" during the Labour leadership election:

"New Labour's proposition was simple – we need to persuade Tory voters to come to us. The task is very different now. Five million votes were lost by Labour between 1997 and 2010. But four out of the five million didn't go to the Conservatives; one-third went to the Liberal Democrats, and most of the rest simply stopped voting."

He was criticised by some for addressing the TUC march because he was preaching to the "already converted". But this is a dangerous assumption to make, one that the Labour party paid heavily for in May last year.

The polling shows that, however large Labour's reservoir of sympathisers is, they cannot be taken for granted. Otherwise they will continue to close doors in the faces of campaigners across the country.