Simon Hughes has launched his bid for the presidency of the Liberal Democrats with a pledge to treble the number of party members by the end of his tenure.
Hughes came third place in the London mayoral contest
The party's ex-home affairs spokesman also predicted the Liberal Democrats, led by Charles Kennedy, would be in government within the next decade.
"We have a great historic opportunity which we have to seize with all our energy," the Southwark MP said.
He faces a fight with the party's Welsh spokesman Lembit Opik for the post.
The pair were the only two candidates to have thrown their hat into the ring by Wednesday's deadline.
Both men are seeking to succeed Lod Dholakia who is retiring.
Launching his campaign, Mr Hughes stressed he was not simply seeking to strengthen his position in the party in order to mount a leadership challenge to Mr Kennedy.
"Contrary to what some newspapers say Charles Kennedy and I actually get on - I like him," he said.
Mr Hughes had cleared his presidential candidacy with Mr Kennedy before putting his name forward, he said.
"It's in nobody's interest to do anything to undermine the leader - we stand and fall by being united," he added.
Montgomeryshire MP Mr Opik launched his campaign for presidency last Thursday when he pledged to motivate his party.
Like Mr Hughes, he denied there was any competition between him and the party leader saying it was his "job to serve Charles".
At his campaign launch, Mr Hughes pledged to end the two-party system within the next two parliaments.
"The job of the president is to make sure that we become the government of Britain," he said.
Mr Hughes, who was comfortably beaten into third place in the recent contest to be London mayor, claims to have the support of two thirds of the Lib Dem Parliamentary Party.
He was backed at his campaign launch in central London by Romsey MP Sandra Gidley, Chairman of the Ethnic Minority Liberal Democrats Fiyaz Mughal and Edinburgh West MP John Barrett.
Mr Barrett said: "I've always been very impressed by Simon's energy and vision for the Liberal Democrats and working alongside Charlie Kennedy and Menzies Campbell I think they would make a great team to lead us into the General Election."
Lembit Ípik won 49% of the votes in his seat at the last election
As part of his campaign Mr Hughes pledged to ensure the number of Lib Dem members, currently 73,000, overtakes the number of Labour Party, now 214,952 in the next four years.
Recent reports suggest Labour membership has halved since Tony Blair became prime minister.
Mr Hughes acknowledged such a leap in membership would require a "significant gear change" but he argued it could be done by turning sympathisers into members and members into activists.
It would mean employing new campaigning methods and new technologies such as text messaging, and pushing for younger and ethnic minority voters, he added.
Ballot papers go out to Liberal Democrat members on 19 July. The result will be announced on 1 September.