MP Chris Huhne has entered the race to be Liberal Democrats leader, bringing the number of candidates to four.
Mr Huhne has been an MP for just eight months
The 51-year-old economics spokesman, in Parliament for just eight months, joins Sir Menzies Campbell, Simon Hughes and Mark Oaten.
"It is a time for new ideas and new horizons" after a "few difficult weeks" for the party, Mr Huhne said at his campaign launch in London.
The MP for Eastleigh spent six years as a member of the European Parliament.
Mr Huhne underlined the importance of environmental issues in his campaign.
He also indicated he would be prepared to work with the other main parties in government in the event of a hung Parliament at the next election.
Simon Hughes - 4/5
Sir Menzies Campbell - 7/4
Chris Huhne - 7/1
Mark Oaten - 8/1
Source: William Hill
Mr Huhne called for an end to the "politics of confrontation" and a focus on "working together trying to achieve what we can across parties".
Having been an MEP would provide "particularly valuable experience" for this, he added.
Former leader Charles Kennedy made Mr Huhne an economics spokesman as soon as he arrived in Westminster and put him in charge of revamping the party's public services policies in 2002.
That had "established my credentials in being able to listen to and pull together those on both the left and the right of the party", he said.
The former City economist and economic journalist added he was "very much in the centre" of a party of "conscience and reform".
But Sean O'Grady, who was an adviser to former Lib Dem leader Paddy Ashdown, told BBC News Mr Huhne's enthusiasm for the euro may see him struggle for support.
His decision to stand follows what has widely been described as a faltering performance by acting leader Sir Menzies at prime minister's questions on Wednesday.
On Thursday, Mr Hughes vowed to unite the party and bring greater success at the ballot box as he launched his bid for the leadership.
Deputy leader Sir Menzies and home affairs spokesman Mr Oaten had already declared they would stand.
25 January - nominations close
6 February - ballot papers sent out
1 March - vote closes
2 March - result announced
Party president Mr Hughes is making his second attempt at the leadership, after being beaten by Mr Kennedy in 1999.
He has replaced former Olympic sprinter Sir Menzies as 11/10 favourite with bookmakers William Hill.
But Sir Menzies, 64, is believed to have the backing of more than a third of the party's 62 MPs.
To stand, candidates must have the backing of at least seven MPs and backing from a range of local associations. Nominations close on 25 January.
Liberal Democrat MPs can nominate more than one candidate.
Mike Hancock, MP for Portsmouth South, is reported to have signed several nomination forms because he wants to ensure a "proper contest".
Shadow Commons leader Theresa May mocked the practice in the house on Thursday, saying "it takes Lib Dem indecision to new heights".
There will be a postal ballot of the party's 73,000 members, with the new leader named on 2 March, a day before the party's spring conference opens in Harrogate.
The candidates will have their first chance to address Lib Dem members directly on Saturday at the party's "meeting the challenge" conference in London.