LONDON — Political rabble-rouser and talk-radio host Nigel Farage announced Friday that his potentially vote-splitting Brexit Party will field candidates for every seat in Britain in the December general election — unless Prime Minister Boris Johnson agrees to abandon the withdrawal deal he negotiated with European leaders and form an alliance with the most zealous Brexit backers.

Farage flung the ultimatum at Johnson at a Brexit Party campaign launch, staged just a few blocks away from the House of Commons, which party chairman and real estate tycoon Richard Tice disparaged as this “stinking, rotten borough of Westminster.”

Farage had already made some election news with a Thursday evening radio interview with a caller from the White House. In that call, President Trump disparaged Johnson’s Brexit plan, saying it could nix a free trade deal with the United States. But he also praised the British prime minister and urged Farage and Johnson to form an electoral pact, saying the duo would be an “unstoppable force.”

Many observers said the U.S. president didn’t seem to know what he was talking about — and that a dynamic duo between Farage and Johnson would ruin the Tory party. 

Johnson on Friday ruled out an alliance with Farage or any other party, “because I don’t think it’s sensible to do that.”

He also delicately dismissed Trump’s assertion about trade.

“I don’t wish to cast any aspersions on the president of the United States, but, in that respect, he is patently in error,” Johnson told Sky News. “Anybody who looks at our deal can see that it’s a great deal.”

But Farage, in his remarks Friday, called Johnson’s deal “a sellout Brexit.” He compared the prime minister to a used-car salesman trying to sell a lemon to a rube.

Farage wants Britain to crash out of the European Union’s trading club without a deal and do business with Europe, its closest economic partner for 40 years, as a “third country” under World Trade Organization rules.

“Boris tells us this is a great new deal. It is not,” Farage said. “It is a bad old treaty and simply it is not Brexit. What we are doing here is kicking the can down the road.”

Further, Farage said, if Johnson and the Tories do not join in a “leave alliance” with the Brexit Party, and make a pact over which seats to contest, then the Faragists will field candidates up and down England, Scotland and Wales. That could lure voters away from the Conservatives and deny Johnson the majority he wants to get his Brexit deal through Parliament.

Asked whether he could — paradoxically — be the last hope for those “remainers” who want to stay in the European Union, Farage said “the risk of the vote being split is very real.”

Farage has never won a seat in Parliament, despite trying seven times. But he is a potent force in British politics. 

His new single-issue Brexit Party won big in the May elections for the European Parliament, taking 31 percent of the vote, far ahead of Labour (14 percent) and the Conservatives (9 percent). It was an impressive showing for a party founded only in January, even if much of its support came in the form of protests votes.

Farage himself was first elected to the European Parliament in 1999 — a body he holds in contempt, though he does accept the comfortable salary, home and travel allowances, pension and money for staffing. In addition, Farage has been awarded housing subsidies, drivers and cars by his Brexit backers.

Farage is the former leader of the U.K. Independence Party, which he abandoned after UKIP swung hard to the extremist right, embracing figures such as Tommy Robinson, a felon and anti-Muslim campaigner.

The Brexit Party brand has allowed something of a refresh.

The Conservative Party line, as chairman James Cleverly put it Friday, is that “a vote for Farage risks letting Jeremy Corbyn into Downing Street via the back door.”

Corbyn is leader of the opposition Labour Party and a committed socialist who surprised many in the 2017 election when his movement surged and denied Conservatives a majority government.

In his radio interview with Farage, Trump dumped on Corbyn, saying he “would be so bad for your country, he’d be so bad, he’d take you on such a bad way. He’d take you into such bad places.”

Many in Britain, both left and right, felt discomfort at hearing an American president so forcefully meddle in domestic politics. Trump’s predecessor, President Barack Obama, was widely criticized, too, for his remarks against Brexit in 2016.

Corbyn tweeted, “Donald Trump is trying to interfere in Britain’s election to get his friend Boris Johnson elected.”

Whatever happens in coming weeks, the next Parliament will look very different.

More than 50 lawmakers are planning to step down, including Ken Clarke, the longest-serving lawmaker in the House of Commons; Rory Stewart, who will turn to trying to win next year’s London mayoral race; Jo Johnson, the prime minister’s brother, who resigned citing “unresolvable tension” between his family loyalty and the national interest; and John Bercow, the flamboyant speaker of the House of Commons.