FOR a constituency that is supposedly safe Labour country, the City of Durham has been doing a remarkable impression of a marginal seat over recent weeks.

On Wednesday morning, prisons spokesman Paul Goggins became the seventh Labour minister to visit the city in the three weeks since the election was called.

He was followed in the afternoon by Lord Wallace, the third Liberal Democrat peer to visit the constituency, arriving hot on the heels of party leader Charles Kennedy.

Even the Conservatives have been getting in on the act.

Bleary-eyed voters in Belmont were woken just over a week ago to find Ann Widdecombe on their doorsteps canvassing for support.

Last autumn, Durham City was the main and somewhat surreal battlefield in the regional assembly referendum, when huge rats scuffled in the streets with inflatable white elephants.

Six months on and voters are once more having to get used to the sights and sounds of modern-day politics in what has emerged as the region's key election seat.

On Tuesday morning, Labour Party chairman Ian McCartney stood on the steps of Durham Prison to condemn what he insisted were Lib Dem plans to give votes to convicted criminals.

Flanked by two Durham University students wearing fake prison uniforms and carrying a polling card in the name of convicted killer Rosemary West, Mr McCartney denied the stunt was a cynical exercise in scaring voters.

Four hours later, Mr Kennedy swept into the University Hospital of North Durham, pursued by a media scrum and both the Labour students, still in their prison uniform, for a tour of the children's ward.

A bewildered middle-aged patient, still attached to a drip, had slipped out of the front door for a quiet cigarette, only to walk straight into the path of a dozen cameramen and party workers in full flight and was almost knocked to the ground, provoking four-letter fury from her family.

Electioneering has returned to Durham.

Outgoing MP Gerry Steinberg bequeathed Labour candidate Roberta Blackman-Woods a majority of 13,400, but since the last election the Lib Dems have won a landslide victory on the city council.

They also came within 500 votes of overtaking Labour at the European elections.

Asked on Tuesday if he was confident of capturing the seat, Mr Kennedy briefly considered his options and answered: "We have reasons to be optimistic - we are campaigning hard to win."

But even if they don't say it in public, Labour seem quietly confident that they can hold on.

For the first fortnight, the party's strategy appeared to centre on shoring up its core vote in the pit villages that surround the city - the vote it needs to turn out on polling day if it is to retain the seat.

Former Home Secretary David Blunkett wowed the pensioners of Brandon during a tour of sheltered housing schemes, Harriet Harman met the mothers and children of Ushaw Moor's Sure Start centre.

At the same time, Lib Dem candidate Carol Woods seemed to be concentrating on galvanising the opposition to Labour behind her banner, appealing to "natural" Lib Dem supporters on issues such as university tuition fees and Iraq, while also targeting the traditionally Tory-voting farmers in the countryside around the city, where every farm now seems to boast a roadside Lib Dem sign.

However, the Lib Dem attack on the city will fail unless it can persuade a significant number of Labour voters to come all the way across and vote for them, rather than simply stay at home.

Consequently, there appeared to be a subtle if significant change of tack about ten days before polling day when both sides were suddenly united in talking about one subject - the Conservatives.

In a message carefully aimed at steadying the nerve of their traditional supporters, both David Blunkett and Ian McCartney raised the spectre of a Conservative Government if they jumped ship.

During his visit on Tuesday, Mr McCartney insisted that Labour were not treating Durham as a marginal.

He said: "The Labour Party isn't scared of the Lib Dems, but we aren't going to take people for granted and we don't want people to go out and vote Lib Dem and wake up on May 6 to the nightmare of Michael Howard in Number 10."

Across the city, Mr Kennedy was highlighting Mr Howard's football analogy that the Conservatives were 2-0 down at half-time, to persuade those same Labour supporters that there was no longer any real danger in voting Lib Dem.

He said: "In this constituency there has been a massive shift of grassroots Labour opinion away from the Government, not just on Iraq.

"People feel confident because they have looked at it, they know there is no prospect of a Conservative Government and they are looking in much more detail at our policies. We are campaigning hard and it is all there to play for."

Certainly it seems both main parties are fighting the campaign like a marginal, not least in the increasingly niggly tone that seems to be creeping in.

Police were called in when 400 of the Lib Dems' roadside signs mysteriously disappeared; Labour fumed when the permission was refused for David Blunkett to visit a council-run old people's home; Labour sailed close to the legal wind in asking whether Carol Woods was lying over votes for prisoners; both sides hinted at behind-the-scenes unrest in their opponents' camp.

Despite the portrayal of the seat as a straight fight between Labour and Lib Dems, the Conservatives insist they are still in the contest. Candidate Ben Rogers - a journalist and human rights campaigner - believes his stance on abortion, human rights and trade justice is winning over some traditional Lib Dem voters, while his campaigns against binge drinking and 24-hour licensing are appealing to traditional Tory voters.

"The response I have had on the doorstep has been really encouraging, there has been a lot more support for me than I expected," he said.

"The Lib Dems have been quite successful in putting out the false message that it is a two-horse race and that might have some effect, but, having said that, I have been pleasantly surprised by the number of Conservative supporters I have met. It is all there to play for."

Four candidates are fighting the City of Durham seat. They are Roberta Blackman-Woods, Labour, Tony Martin, Veritas, Ben Rogers, Conservative, Carol Woods, Liberal Democrats.