Senior Lib Dem MP Simon Hughes has entered the race to replace Vince Cable as deputy party leader.
The Bermondsey and Old Southwark MP, a former Lib Dem president, is backed by Mr Cable who resigned to focus on being business secretary.
He described himself as "one of the guardians of the soul of the party" and vowed to protect its independence while it is in coalition with the Tories.
Tim Farron is also standing.
Mr Farron has the backing of former party leader Sir Menzies Campbell.
'Life and soul'
Mr Hughes, who stood for the Lib Dem leadership in 2006, has held frontbench roles but was not given a ministerial job in the coalition government.
He describes himself as being on the "radical" left wing of the party, and said he aimed to ensure the Liberal Democrats remained a supportive but independent voice in the coalition government.
He told BBC News: "I would see myself as a guardian of the soul of the party," adding that he wanted the party to win power in its own right one day and to replace Labour as the "progressive centre-left force in British politics".
He added: "I want the job because I believe that it's very important that the deputy leader is from the parliamentary group of Liberal Democrats who are not in government.
"So that people understand that we are an entirely separate, independent, proud political party which for the time being is in coalition.
"We are currently doing better than we have done in our lifetime and we have a huge contribution to make in British politics.
"I want us before I finish my career to be the leading party of government in the UK and there's no reason why we can't do that"
He said his campaign slogan was "the life and soul of the party".
Candidates must get the backing of five of the party's 57 MPs by 2 June to go through to the ballot. The new deputy will be elected on 9 June.
Mr Hughes, 59, was also the Lib Dems' candidate for London mayor in 2004 and has been an MP for 27 years.
He was backed by Mr Cable who said he would "uphold the values of our party" and had given "the most phenomenal service to the party".
He added: "He represents the best traditions of the Liberal Democrats, both as a parliamentary campaigner and community activist... Simon is the person best placed to follow me as deputy leader and uphold the values of our party."
Mr Cable was elected deputy leader in 2006, defeating former MP Matthew Taylor and current Lib Dem minister David Heath.
He became acting leader after the resignation of Sir Menzies Campbell in 2007 but opted not to run for the leadership at that time, citing his age as one factor.
Mr Cable helped raise the profile of the party during the financial crisis, having been one of the first senior MPs to draw attention to the risks of the lending boom.