Boris Johnson promises to lead the United Kingdom out of the European Union on October 31 "do or die."

The favourite to replace Theresa May as prime minister has taken a hardline Brexit stance in order to win over the overwhelmingly pro-Brexit Conservative party membership.

However, anti-Brexit campaigners say Johnson's rhetoric has energised opponents of Brexit.

They report significant increases in donations and support from Members of Parliament.

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LONDON — The prospect of Boris Johnson becoming the United Kingdom's next prime minister is energising the campaign to stop Brexit, with campaigners reporting significant increases in donations since he began his leadership campaign.

The favourite to replace Theresa May has promised to take the UK out of the EU "do or die" on October 31, and has refused to rule out shutting down parliament in order to force through Brexit, as he bids for the votes of the overwhelmingly pro-Brexit Conservative membership.

However, anti-Brexit campaigners say that Johnson's hardline rhetoric has energised potential supporters both inside and outside the House of Commons into rallying behind their cause.

A senior figure in the campaign for a new referendum said that Johnson was "concentrating the hearts and minds" of MPs who previously had not backed the campaign for a second Brexit referendum.

MPs in both the Conservatives and Labour who "really, really hate Boris are more likely to have chats with us," the source told Business Insider, adding that talks were underway with increasing numbers of MPs who are considering backing a new referendum.

The prospect of Johnson leading the UK has also sparked an increase in small donations from supporters, campaign sources say, as "like [Nigel] Farage, he is a totem on the other side that people rally against."

Recent opinion polls suggest that while Johnson is popular among Conservative and Leave voters, he is deeply unpopular among more liberal and anti-Brexit voters.

In Farage's pocket

On Friday, activists from the official People's Vote campaign for a new referendum descended on Exeter, southwest England, where Johnson participated in a Conservative leadership hustings with his rival, Jeremy Hunt.

They unveiled a poster showing Johnson in the pocket of Brexit Party leader Farage with the caption "in his pocket."

A People's Vote source said: "We're leaning more towards criticising Johnson because his views on Brexit are further to the more extreme end than Hunt — but both have have fantastical, unicorn-ist views on what's achievable."

As well as Johnson's Brexit policy, the People's Vote campaign is focusing on what they argue is the democratic illegitimacy of having such a small percentage of the public choose May's successor.

The Conservative party membership — approximately 160,000 members — makes up just 0.25% of the UK population, but will have sole responsibility over choosing the country's next prime minister.

People's Vote figures — who are currently touring the country as part of their Let Us Be Heard campaign — are focusing on "drumming up a sense of democratic outrage" over an unelected prime minister attempting to force through Brexit.

"Conservative party members are entitled to pick next leader of the Conservative party. What we have a problem with is that this tiny group of people will decide where we end up on Brexit," a People's Vote campaign source said.

"Our campaign is focused on everyone having a say on where we end up on Brexit."

The campaign is set to visit both Remain and Leave-voting locations around the country like Cheltenham and Sunderland, before marching in London October 12 — just over two weeks before the UK's scheduled exit date.