Hardline Brexit advocate Nigel Farage opened his UK election campaign on Friday by telling Conservative prime minister Boris Johnson that his Brexit Party will contest every seat unless Johnson drops his EU divorce deal and agrees an election pact.
The call was swiftly rejected by Johnson and his party.
The snap election, set for December 12, is highly unpredictable so an alliance on either side of the Brexit schism could be a game-changer after nearly four years of political crisis over Britain's decision to quit the European Union.
Farage cast his proposal as a non-aggression pact.
"I will say this to Boris Johnson: drop the deal because it is not Brexit, drop the deal because, as weeks go by and people discover what it is you will have signed up, they will not like it," he told reporters at the launch. "This is not Brexit."
"He is trying to sell a second-hand motor where he has polished up the bonnet but actually, underneath, nothing has changed, and it is Mrs May's appalling surrender treaty," Farage added. Johnson's predecessor May agreed to a less radical divorce with the EU but had it rejected by parliament three times.
Farage has shown in the past that he can unnerve Conservative leaders with the threat of poaching their voters.
Opinion polls give Johnson a healthy lead over the main opposition Labour Party, but also suggest that more than 10% of voters back the Brexit Party - enough to split the pro-Brexit vote in some seats and hand victory to Labour.
Farage said that if Johnson rejected his proposal, the Brexit Party would fight for votes in every seat.
Instead, he proposed to stand aside in around 500 seats in return for the Conservatives giving his party a clear run in about 150 seats where he thought the Brexit Party had a better chance of winning. He gave Johnson until Nov. 14 to consider.
As the leader of the UKIP party, Farage helped force May's predecessor David Cameron to call the Brexit referendum in 2016 and then played a leading role in the campaign to leave the EU.
The idea of a pact was endorsed by US President Donald Trump - a friend of Farage - who called up Farage's radio show on Thursday and said: "If you and he get together it's, you know, unstoppable force."
But the Conservative Party showed no sign of dropping its oft-stated refusal to make any election pact.
"It will not get Brexit done - and it will create another gridlocked parliament that doesn't work," party chairman James Cleverly said.
Johnson later ruled out dropping his Brexit deal.
"What we've got is a fantastic deal that nobody thought we could get," he told ITV television. "We can put that deal through."
Shunned by the political establishment, Farage, backed by wealthy eurosceptic financiers, helped sell Brexit to millions of voters in England and Wales who felt ignored by the mainstream parties.
His enemies say he sells a nostalgic vision of a Britain that never was to voters made insecure by the effects of globalization, though opponents admit that their failure to address concerns about immigration allowed him to build support.
He proposes what he calls a "clean-break Brexit", which means the type of "no-deal" departure, with no arrangements to temper the economic shock, that many international businesses and banks say is their nightmare scenario.
Farage urged Johnson to abandon transition arrangements or hopes of long-term political alliance with the EU and instead pursue a free trade deal.
He said Johnson must set a July 1 exit deadline and leave under basic World Trade Organisation terms if such a deal could not be agreed.
Farage told Reuters that Johnson had shown his ability to change position - "and now, Mr Johnson, it's the time to drop the treaty and get on with a proper Brexit".