Lib Dem campaign takes off in vital marginal as Labour and Tory voters defect

If the nation's sudden love affair with Nick Clegg lasts until polling day the Liberal Democrats are on track to pick up a three-way marginal being strongly targeted by the Conservatives, a survey by The Independent suggests.

A straw poll of 250 likely voters across the Hertfordshire constituency of Watford found the Lib Dems with a commanding eight-point lead over the Conservatives. When the results are weighted to take into account the outcome of the 2005 election the margin of victory triples. In 2005, the three parties were separated by fewer than 1,000 votes each, and it is a vital target seat if the Conservatives are to win an overall majority.

The survey found that 36 per cent of voters said they would vote for the Liberal Democrats' candidate, Sal Brinton, with the Conservatives second on 28 per cent. The Labour incumbent Claire Ward is set to pick up just 20 per cent of the vote – little over half of what she received in the last election.

The Liberal Democrats' gains come at the expense of both the Conservatives and Labour. One in five voters who backed the Tories in 2005 say they plan to defect, with many citing Nick Clegg's widely praised debate performance as a factor. One in three Labour voters is also set to make the switch.

There was already a "buzz" to the Liberal Democrat campaign before Nick Clegg's appearance in the leaders' debate, Ms Brinton said, but since then, she says, it has really taken off. "People have been walking in off the street asking for posters, saying 'Don't bother asking, you've got my vote' and giving us the thumbs up in the street. It really has felt great'."

Watford's Conservatives have not had a good time since the last election. They selected a candidate named Ian Oakley, who was arrested, charged with 75 offences of harassment, criminal damage and intimidation against the Liberal Democrats and given an 18-week suspended jail sentence. David Cameron apologised for his behaviour during a visit to Watford in March last year.

Richard Harrington, adopted in Mr Oakley's place late in 2008, has done his best to revive the local Conservative organisation, and claims that he now has 200 volunteers helping his campaign, many of whom were not old enough to vote in 2005.

Claire Ward speaks warmly of her Tory rival as a "nice man" – a sure indication that Labour does not see him as the main threat. The reason is shown up in recent local government elections. Watford's directly elected mayor, Dorothy Thornhill, is a Liberal Democrat. Of the councillors representing wards in the constituency, 40 are Liberal Democrat, four are Conservatives, three Labour and two others.

Ms Ward's advantage is that, having become Britain's youngest woman MP at the age of 24, she has represented Watford for most of her adult life.

Additional reporting: Billy Kenber, Stephen Morris, Tom Brooks-Pollock, Rob Hastings and Alan McGuinness