Business secretary Vince Cable said the Eastleigh by-election is an opportunity for Liberal Democrats to highlight "the many differences" between the coalition partners.
Speaking from the Hampshire constituency where he is helping the Lib Dem campaign he told the Press Association: "It the first major opportunity we have had since the coalition was formed to go head-to-head and I know that our activists will relish the opportunity."
Mike Thornton, a parish and borough councillor since 2007, is the Lib Dem candidate and is the favourite to win. The Lib Dems held it in the 2010 General Election with a majority of 3,864.
Cable said he was confident of Lib Dem success in seeing off the Tory challenge of Maria Hutchings, a businesswoman and mother-of-four, in what many think will be a close contest.
Cable told the Press Association:"We are different parties and we believe in many different things and a by-election, particularly around fairness in taxes, is an opportunity to highlight elements of those differences without jeopardising the stability of the government - we have no wish to do that."
Cable pledged it would not be a dirty fight as long as the Conservatives did not start it.
"It does not need to be (a dirty fight) and it should not be," he explained.
"If people's confidence in politics is to be maintained we need to make out case in a dignified and sensible way.
"We will fight a straight forward campaign. The candidate is a honest, straight forward candidate."
On the demise of the former MP Chris Huhne who awaits sentence after pleading guilty to trying to dodge a speeding fine, Cable said: "It's a terrible tragedy for the family and that's the thing that has come across - it's an awful personal tragedy."
But when asked if he felt betrayed by Huhne and whether the disgraced politician had professed his innocence to him, Cable said: "He did, yes, but Nick Clegg has spoken about this on our collective behalf and this is now a matter for the courts of law it is not a political matter."
Thornton was also admitted the Lib Dems were in the box seat but he vowed not to be complacent.
"Even when we have 40 out of the 44 seats on the borough council, we know we have to come up with the right policies for the people of Eastleigh because they are hard to please."
He said that he was pro gay marriage - unlike his Tory rival - and said that in the 21st century he was "astonished that anybody should not have the equal chance of an equal loving relationship".
He explained that he felt that Chris Huhne's disgrace would "not particularly" affect the Lib Dem vote.
"What we are talking about is our record in government and our record locally. I know the team in Eastleigh is honest and straight forward. People in Eastleigh borough trust us and we are totally open," he added.
Meanwhile polls on who will win show the contest will be close.
The Survation poll for The Mail on Sunday put the Lib Dems on 36%, ahead of the Tories on 33%. The poll also put the UK Independence Party on 16%, ahead of Labour on 13%.
Survation polled 504 people in Eastleigh by telephone on Thursday.
There was some excitement in Labour circles today as the author and broadcaster John O'Farrell announced that he was going to put himself forward to be the party's candidate in the by-election.
In a move which some supporters thought would significantly improve Labour's chances of competing with the Tories and Lib Dems for the seat, O'Farrell took to Twitter to declare his interest in running.
Labour is expected to select its candidate on Tuesday.