As the Brexit party’s future stood in the balance, Nigel Farage sought comfort in a boxing ring in the old Derbyshire mining town of Bolsover.

The constituency voted to leave by more than 70% – one of the highest leave votes in the country – making it ideal territory for Farage as he set out on his bid to win historically Labour-voting seats from Jeremy Corbyn.

In the May local elections this year Labour’s 40-year dominance over the town – so strong that its famed veteran MP and lifelong Eurosceptic, Dennis Skinner, was nicknamed the Beast of Bolsover – was well and truly overthrown. The party suffered one of the biggest defeats in its history, losing control of the local district council for the first time since its inception.

With Bolsover castle looming in the background, about 150 Brexit party supporters piled into the boxing ring in an industrial estate on Tuesday holding up banners with the tag line, “Change politics for good”.

They watched as Farage, wearing a pair of red boxing gloves, mock sparred with the cameras before taking swipes at both Boris Johnson and Jeremy Corbyn, saying his candidates were the “only people offering Brexit” and denying any intention to split the Conservative vote. He also said the party would keep its promise on Brexit and accused Corbyn of betraying his working-class supporters.

Brexit party supporters listen to Nigel Farage. Photograph: Christopher Thomond/The Guardian

Despite his bluster it had been a bruising 24 hours for Farage. On Monday, at a lavish campaign launch in central London at which hundreds of new parliamentary candidates were paraded before the media, he committed the Brexit party to standing more than 600 candidates in the general election.

However, just hours later his plans seemed to be in disarray as it emerged that 20 candidates had already quit. In the marginal Dudley South seat, Paul Brothwood stepped down and gave his backing to the incumbent Conservative MP, Mike Wood. In the Kent constituency of Tonbridge and Malling, Stephen Peddie announced his resignation by publicly attacking Farage on Twitter, accusing him of pursuing a “fantastical and dangerous strategy”. Meanwhile, Dan Day-Robinson, who was standing in Devizes, Wiltshire, decided to stand down for personal reasons as his partner is pregnant.

Last week the party confirmed that Jill Hughes, the candidate for the Batley and Spen constituency in West Yorkshire, was to stand down after claims emerged that she believes she comes from the distant star Sirius and that governments across the world were in cahoots with aliens.

At the same time, one of Farage’s main donors announced he was ditching the party and switching his support to the Conservatives, warning that Farage risked hindering the Brexit cause.

Jeffrey Hobby, who donated £10,000 to the Brexit party ahead of the European parliament elections in May, said: “It is a shame if Nigel Farage wants to take him [Boris Johnson] on across the country. I don’t think that is helpful for the Brexit party, the Tory party or the cause.

“For the Brexit party to field candidates across all seats seems a waste of resources and not the wisest move.”

Back in the boxing ring, three candidates including Kevin Harper, a former police officer who is standing in Bolsover, and mother-of-three Debbie Soloman who is standing in Bassetlaw, took to the stage outlining their campaign promises before handing over to Farage.

As he bounded on, his first blow was aimed at the government, accusing them of a series of broken promises to the people of Bolsover and failing to deliver the Brexit they had voted for, which had “eroded trust in politicians”.

He said he had decided not to stand in the general election so he could “traverse the length and breadth of the country” to help potential candidates get elected.

Asked what the Brexit party’s realistic aims were for the 12 December poll, he said: “To offer people a choice. They deserve to vote for a candidate who says we should leave the European Union, leave its institutions and not be trapped for years and years to come.

“Those who want a clean-break Brexit, and that’s what 17.4 million people said, the vote to leave was to leave the EU, not to stay trapped to it.”

The “gloves were off” in the campaign, he continued, and he would be going after Corbyn’s Labour with “5 million” Labour leave voters his key target, while also urging Johnson to change course.

Discussing Johnson’s strategy, he said: “Look, perhaps he doesn’t want a pact because he wants to continue with his EU treaty. He wants us to be half in, half out, and go straight into three more years of expensive negotiations where we are tied in every way.

“I would urge the prime minister to change course. He did what he could in a very difficult situation he inherited. It’s a very remain parliament. The general election is a chance to press the reset button. At the moment he has chosen not to. If that’s the case, I will stand for Brexit if he doesn’t really want to.”

• This article was amended on 6 November 2019. An earlier version misspelled the constituency of Bassetlaw as “Basset Law” and the last name of the Brexit party candidate Debbie Soloman as “Solomon”.