Mark Oaten has withdrawn from the contest to become the next Liberal Democrat leader.
Mr Oaten lacked support among MPs
Mr Oaten, the party's home affairs spokesman, said the other contenders all "had much more support" among MPs.
His withdrawal leaves Sir Menzies Campbell, Chris Huhne and Simon Hughes as the remaining candidates.
A new Lib Dem leader will be announced on 2 March. He will replace Charles Kennedy, who resigned earlier this month after admitting a drink problem.
Mr Oaten, the 41-year-old MP for Winchester, said he still wanted to make the Lib Dems a "21st century party".
Announcing his withdrawal, he added: "I wanted to stand because I felt it was very important to have a contest."
But support among party members had not been "matched by my colleagues in Parliament, apart from the very loyal support I've had from Lembit Opik".
Mr Opik, Lib Dem Northern Ireland spokesman and Mr Oaten's campaign manager, would not say whom he now supported.
In Parliament since 1997, Mr Oaten is seen as being to the right of many in the party, having promoted a law and order policy of "tough liberalism".
Lib Dem Europe spokesman Nick Clegg, a supporter of deputy leader Sir Menzies Campbell, told the BBC Mr Oaten had come "to a mature decision ... clearly the support among MPs was never there".
Asked if he expected Mr Oaten to now come out in support of Sir Menzies, he said: "We haven't had any contact with him yet, but he would make an extremely strong member of the front bench team."
Mr Oaten was the second MP to announce his candidacy, after Sir Menzies.
Party president Mr Hughes and Treasury spokesman Mr Huhne then joined the contest.
Mr Opik said: "It was Mark's declaration that he was going to stand that ensured there was a contest.
"The others came in after him. It was he that ensured there was a choice for the membership."
Mr Huhne said he regretted Mr Oaten's decision.
"While I understand his reasons for withdrawing, this does mean that party members have less choice over the leadership of the party," he said.
The MP said he was sure Mr Oaten would continue to play a leading role at the top levels of the party.
That view was echoed by Mr Hughes, who called Mr Oaten "a great asset to the party" who would have "an important role to play in my front-bench team".