UK Independence Party deputy leader Mike Hookem has quit his position and will run for party leader.
Mr Hookem said he wanted to offer "a real alternative" to the leadership of Gerard Batten.
Mr Batten was elected to the top job unopposed in April 2018, and promised to resign after 12 months to allow a full raft of candidates to run.
In a letter to party officials, Mr Hookem said the leader had "squandered our natural advantage".
"I believe UKIP always has been and always should be a libertarian party that encourages and promotes common-sense policies with a broad electoral appeal," he wrote.
"However, under Mr Batten's leadership, and despite my appeals, UKIP has been derailed from this objective.
"Mr Batten's policy direction and associations have given the mainstream media the ammunition to label our party 'extreme' and 'far-right', accusations I do not believe to be true."
Mr Batten has been criticised for the direction he has taken UKIP, especially after naming former English Defence League leader Tommy Robinson - whose real name is Stephen Yaxley-Lennon - as an adviser.
But Mr Batten said he had saved the party "from oblivion", and that Tommy Robinson was "a good person to have onside".
Mr Hookem, who's MEP for Yorkshire and the Humber, said he had been "desperate to act to stop the increasing isolation and marginalisation of our party in recent months.
"However unlike some who found alternative parties, I stayed loyal to the membership and delayed my resignation to minimise any potential electoral impact."