Sir Ed Davey and Layla Moran will fight it out to become the next Liberal Democrat leader after securing the support required to stand.
Both candidates obtained the necessary backing from 200 members in 20 constituency parties and one other MP after nominations opened on Wednesday.
The only other declared candidate, Wera Hobhouse, pulled out of the contest on Tuesday and is backing Ms Moran.
Voting starts next month, with the result to be announced on 27 August.
Sir Ed, who was a cabinet minister in the coalition government between 2010 and 2015, has been serving as acting leader since Jo Swinson lost her seat in the 2019 general election.
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He is being challenged for the top job by Ms Moran, the party’s education spokeswoman who has been an MP since 2017.
Both candidates posted messages on social media saying they had been told that they had passed the threshold for nominations, thanking their supporters.
The final line-up will not officially be known until 9 July, when nominations close, but no other candidates are expected to enter the race.
The Lib Dems originally planned to delay the contest to succeed Ms Swinson until May 2021 but brought it forward following criticism from party members.