Outgoing UK Independence Party leader Nigel Farage has backed Lord Pearson as the only "serious, credible" candidate in the race to succeed him.
Mr Farage, who is standing down to run against Commons Speaker John Bercow at the general election, said the peer was "head and shoulders" above his rivals.
He told BBC Two's Daily Politics any other other winner would make UKIP's situation "tricky".
There are five candidates in the leadership race.
The other hopefuls are MEPs Gerard Batten, Mike Nattrass and Nikki Sinclaire, and Alan Wood, a councillor from Wiltshire.
Mr Farage, who will remain as UKIP's leader in the European Parliament, said: "There are five candidates standing. Only one of them is a serious, credible candidate and that's Lord Pearson, who has had major achievements in his life in business and politics too.
Lord Pearson defected from the Tories in 2007
"If Lord Pearson gets that job and I'm leading the party in the European Parliament, then I would argue that UKIP is stronger and will be for several years to come.
"If it's not Lord Pearson, things will be tricky. But I think it will be Lord Pearson. He is head and shoulders above the others."
Lord Pearson, a former insurance broker who was ennobled by Prime Minister Margaret Thatcher in 1990, was one of two Conservative peers to defect to Mr Farage's party in 2007, saying UKIP was the "only party telling the truth" about Europe.
He hit the headlines this summer when he invited controversial Dutch MP Geert Wilders - who is calling for the Koran to be banned in his country - to the House of Lords.
The Home Office turned Mr Wilders away from the UK.
But he was later allowed in when the Asylum and Immigration Tribunal ruled there was no evidence to suggest he represented a real and serious threat to the "fundamental interest" of society.
In his manifesto, Lord Pearson advocates an "amicable divorce" from the European Union and promises to be a champion of freedom of speech.
Leadership ballot papers were sent to UKIP members earlier this month and they have until 26 November to vote.
The party, which wants the UK to withdraw from the EU, came second in June's European elections, taking 13 seats.
Mr Farage will stand against Mr Bercow, formerly a Tory MP, in his Buckingham seat at the next general election.
He said he was resigning as leader to concentrate on this, adding that he was "not running away" or "deserting", as the party had "never been more successful".
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