THERE'S THAT "OH HELL" MOMENT, a mix of fear and hesitancy, that spreads across a leading politician's face when they're interrupted  during a high street walkabout. On his soapbox in Nantwich yesterday, you could sense Jack Straw didn't know what was coming.

A bald man, wearing a football strip and with a bull terrier in tow, pointed his tattooed arm at Straw and broke into the justice secretary's speech, shouting: "I'll tell you this. I was in hospital and at 7 o'clock in the morning and I didn't have a cup-holder. I called Gwyneth Dunwoody, and by 11 o'clock I had my cup-holder. That's the kind of MP I want. My name's Trevor."

"I promise he's not a plant" laughed Straw. "You're a great man Trevor."

This is as civilised as Labour's campaign in Crewe and Nantwich gets. Since the death of Gwyneth Dunwoody last month, who held the Cheshire constituency for 34 years, Labour have mounted a by-election campaign dripping in class-war rhetoric that, if it works, could be the prototype of the looming battle to keep David Cameron out of Number 10.

In Labour's campaign HQ six miles away in Crewe, there's no apology from party activists for their actions. Tamsin Dunwoody, Gwyneth's daughter, is Labour's attempt at hereditary representation. Folding posters that call for the Dunwoody lineage, with backing from Sir Alex Ferguson and Coronation Street's Liz Dawn, two party workers nod to each other. "She is one of us, that's a fact. It's one of us - or one of them."

"One of them" is the Conservative candidate Edward Timpson, a barrister. His public school background and link to the Timpson shoe empire has seen Labour try to turn him into Lord Snooty. With activists dressed in tail-coats and top hats, choreographed by the whip and Birmingham MP, Steve McCabe, Labour's message is that "Tory Boy Timpson" can only let Crewe's working-class voters down because he's no idea what it's like to live on a low income.

With PG Wodehouse's Come On Jeeves currently playing at Crewe's Lyceum theatre, an elderly local couple, Stan and Barbara, said there was no need to see the play: "The comedians, the politicians, are all over the bloody place these days. We don't need a top hat to help us tell who is a Tory. Since Tony Blair we're all bloody Tories."

Disregarding this view, Crewe and Nantwich is a constituency that, post-Blair, reflects a convenient class split Labour hope will work in their favour.

Crewe was the location the Grand Junction Railway company chose in the 1830s to build its giant locomotive works and railway station, and a small village found itself at the centre of a heavy industry revolution. The town prospered in the good times and suffered in recessions. Nantwich, the only Cheshire town to support Cromwell's forces during England's civil war, was first offered the rail works, but turned it down. It has since remained the more prosperous, quieter end of the constituency, more Bournemouth than Blackburn.

In 1983, at the height of Margaret Thatcher's popularity, Gwyneth Dunwoody's majority was just 290. Successive general elections saw this rise to just over 1000, then 2695, and in 1997, at the dawn of New Labour, her majority was nearly 16,000. At the last general election it was down to 7000.

Recent polls point to Tamsin Dunwoody being unable to replace her mother, and some in Timpson's team believe he can be in Westminster next week with a 2000-plus majority.

Timpson's campaign, which has included a visit by David Cameron, is exposing a Labour strategy that seems to be looking back to class division rather than the once-inclusive message of Blair's New Labour. Labour's posters showing "Tory Boy Timpson's mansion house" ignore Timpson's real history, which saw his parents adopt more than 80 children, bring two youngsters formally into their family, and see Timpson himself go on to specialise in family law and child protection.

One Labour MP who has refused to go to Crewe, said: "Toffs versus the proles is something I thought we'd got over."

MP Ian Austin, Gordon Brown's former spin doctor at the Treasury and now the prime minister's principal private secretary, was in Crewe last week. Austin has no problem with the way McCabe is running things. "We are focusing on the Labour vote. It's about ensuring that they've got the message about what we did this week."

Austin was campaigning with the Treasury minister, Angela Eagle. She said they had been concentrating in Labour areas and she hadn't gone to "the posh bits". Austin and Eagle are part of stream of senior Labour visitors but, according to Austin, Gordon Brown won't make an appearance in the three days left of the campaign.

Others in the Cabinet are said to be unhappy about the class-war tactics. When Harriet Harman visited Nantwich last week, Labour's deputy leader said that Dunwoody "always felt Nantwich was just as important as Crewe."

But on the main routes between Crewe and Nantwich, there is a sea of posters for Timpson. Tory MP Mark Pritchard, said: "The Conservatives are no longer a disliked brand and that means people in this campaign have come to us offering support and asking to work for the party."

Back in Nantwich, Straw was making last-minute conversion. "You're one of the true old Labour ones!" a woman shouted. Straw didn't know whether to thank her or not. Old Labour were good at losing elections. Then the Tory supporters appeared, shouting "Get Brown out - remember the 10p tax."

Party supporters around Straw replied with: "Bye bye, toffs!"

None of it much impressed Noel Lee. Lee had just told Straw his reservations on MPs' expenses, how Blair "was there" but Gordon's not, that Brown was dour.

"You see where I'm coming from, Jack?" Jack said he did, but hoped Labour and Dunwoody had done enough for schools and the NHS over the last decade to hold on to Lee's vote.

So had Straw had told him what he wanted to hear. "Labour are no longer ... well Labour," said Lee. So if he wanted them to be more Labour, did that mean he wouldn't be voting Conservative? "Well, I haven't decided yet. I might." He has till Thursday to make up his mind.