Simon Hughes vowed to unite the Liberal Democrats and bring greater success at the ballot box as he launched his bid for the party leadership.
Speaking at the Oxo Tower in London, Mr Hughes said the Lib Dems' record result at the 2005 election should be a "stepping stone to greater progress".
Deputy leader Sir Menzies Campbell and home affairs spokesman Mark Oaten have already declared they will stand.
New MP Chris Huhne has also declared his intention to stand.
The Lib Dem economic spokesman, a former MEP, will launch his bid to succeed Charles Kennedy on Friday.
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.
Grass roots support?
Party president Mr Hughes is one of the best known Liberal Democrats, and stood to become London mayor in 2004, but lost when Ken Livingstone was re-elected.
He is making his second attempt at the leadership, after being beaten by Mr Kennedy in 1999.
He said he believed he would not be standing if he did not think he could win.
Seven out of 10 Lib Dem members had last year voted for him to become their party president.
25 January - nominations close
6 February - ballot papers sent out
1 March - vote closes
2 March - result announced
He was now asking the same people to propel him into the top job.
Speaking without notes, the 54-year-old Bermondsey MP said: "What I have to offer is my ambition, enthusiasm and passion.
"What I have to offer is my experience over many years in Parliament and campaigning around the country to motivate people to join us."
Mr Hughes is sometimes branded as a left-winger but he insisted his views fitted in the mainstream of British politics.
The Lib Dems had made the right judgement on Iraq and Europe, had defended civil liberties and the environment.
But it now had to show it could meet other public concerns, including their homes, debts, pensions and security.
Mr Hughes argued his party had to overcome a credibility gap among people who liked its policies but did not think the Lib Dems could win.
He has been nominated by parliamentary party chairman Paul Holmes, education spokesman Steve Webb, ex-health spokesman Evan Harris, Annette Brooke, Tim Farron, Mark Hunter and Richard Younger-Ross.
Mr Hughes 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 and has support from ex-leaders Lord Steel and Lord Ashdown.
The only other confirmed candidate so far is Mr Oaten, who is seen as more to the right of the party.
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 in the contest.
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.
Lib Dem MPs and peers held what was said to be an emotional four-hour meeting on Wednesday.
They discussed the departure of Mr Kennedy, who resigned on Saturday after admitting to a battle with drink, and the way forward for the party.