Nominations in the race to become the next Lib Dem leader close on Friday at 16:00 BST.
So far two candidates have announced their intention to run – deputy leader Jo Swinson and former Energy and Climate Change Secretary Ed Davey.
Announcing his resignation in March, leader Sir Vince Cable said he wanted to pave the way for “a new generation”.
The winner will be decided by a postal ballot of party members and the results are due to be announced on 23 July.
Candidates need support from 10% of the party’s 11 MPs and 200 members from at least 20 local parties in order to run.
Sir Vince became party leader without a contest after Tim Farron’s resignation in 2017.
Under him, the Lib Dems have called for another EU referendum as a means of stopping Brexit – and joined forces with pro-referendum campaigners in other parties in the People’s Vote campaign.
Until recently, the Lib Dems struggled to get out of single figures in opinion polls. But the party achieved good results in English local elections in May and came second in the UK’s European elections three weeks later, winning 20.3% of the UK vote.
2015Lost seat in election after 5 years of coalition government
2005Elected MP at age 28 after working at Viking FM in Hull
2018First MP to take her baby into a Commons debate
Announcing her candidacy, Ms Swinson said the country was “crying out” for a movement “to challenge the forces of nationalism and populism”.
“The Liberal Democrats need to be at the heart of that movement and I’m the person to lead it,” she added.
2016Knighted for “political and public service”
2012-2015Energy secretary in the coalition government
1997First elected MP after career as management consultant
Setting out his plans, Sir Ed said the country needed “a new economic model”.
He added: “I’m talking about de-carbonising capitalism, making capitalism turn green so Britain is a world green finance capital.”