William Hague says Tory candidate never advocated building in green belt
MPs from all three major political parties are expected to take to the streets of Henley to campaign ahead of Thursday's by-election.
The 26 June poll was sparked by the resignation of Tory MP Boris Johnson following his election as London mayor.
It will be the first since the Tories triumphed at Crewe and Nantwich last month and a major test for party leader David Cameron, MP for nearby Witney.
The Lib Dems were the nearest challengers at the 2005 election.
Party leader Nick Clegg and shadow foreign secretary William Hague are among the big names out and about in Henley on Monday, which is considered one of the safest Conservative seats in the country.
Liberal Democrat leader Nick Clegg says his candidate is the best man for the job
Mr Hague said John Howell, the Tory nominee, was "fighting a positive campaign", adding that he was the only local candidate of the three main political parties.
"We take nothing for granted - there is no complacency in the Conservative campaign," he told the BBC News Channel.
Mr Clegg, who is on his seventh visit to the constituency, told the BBC his candidate, Stephen Kearney was "fighting to win".
He defended the fact Mr Kearney was from Plymouth, saying the local party had "selected the best person" for the job.
"We are now campaigning very hard, right up to the finishing line this Thursday - we're fighting to win," he said.
Labour's candidate may win on schools and hospitals, says O'Farrell
John O'Farrell, Labour activist, author and broadcaster, said it was "quite possible" his party would lose its candidate's deposit.
"It depends whether traditional Labour support thinks it's worth coming out - I hope they do," he told the BBC.
UKIP leader Nigel Farage said: "I think we have got a very good chance here of beating the Labour Party into fourth place."
The list of candidates nominated for this Thursday's by-election, according to the returning officer, are:
Chris Adams, UK Independence Party
Derek Allpass, English Democrats
Louise Cole - Independent (Miss Great Britain Party)
Amanda Harrington - Independent (Miss Great Britain Party)
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