The most astonishing moment of Boris Johnson’s speech on the fringe of the British Conservative Party conference was his thinly-veiled suggestion that the compromise plan on Brexit backed by Prime Minister Theresa May is flirting with treason.

The former foreign minister, laying the ground for his own bid for leadership, stopped short Tuesday of directly accusing Mrs. May & Co of having committed treason. He didn’t even use the T-word. He skated, though, close to the line.

Were Britain to leave the European Union and relinquish membership rights, yet remain subject to evolving trade regulations as part of the Chequers compromise, Mr. Johnson said, its authors would “risk prosecution under the 14th century statute of præmunire, which says that no foreign court or government shall have jurisdiction in this country.”

That was a nod to a series of laws designed to block papal authority, bowing to which was seen as an act of treason. The laws of præmunire are now defunct. The point, though, put into the sharpest relief yet the rift among the governing Conservatives.

Mr. Johnson gave his much-heralded speech at the Conservative Party autumn conference at Birmingham. He sought to make light of the speech’s impact by noting “this is only a fringe meeting, unlikely to be widely reported.”

It’s been all over the newswires ever since. Before an overflow audience, BoJo, as he is called, presented in effect an “alternative leader’s speech.” He addressed anxieties about Britain and its global role. Striking out at socialism, he hewed to the verities of conservative belief.

The shaggy-haired, bicycle-riding ex-mayor of London shared his personal fears, first for the Conservatives — “that after 200 years this oldest and most successful of all political parties should somehow lose confidence in its basic belief in freedom.”

Then for Britain, “that after 1,000 years of independence this country might really lose confidence in its democratic institutions.” Mr. Johnson marked the fear animating Brexit, that Britain is “so demoralized and so exhausted as to submit those institutions — forever — to foreign rule.”

“Only a strong private sector can pay for superb public services,” he averred, crediting “the central symmetry of our one nation Toryism.” That’s a phrase inspired by Disraeli’s evocation, in his novel “Sybil,” of two nations, “the Rich and the Poor,” and of lifting up the poor through liberty and enterprise.

BoJo’s encomium to prosperity echoes not only Disraeli but also President Trump, from praising innovators and entrepreneurs to denouncing regulation’s stranglehold over growth.

“When I champion the market economy you can see that I do not claim that it is perfect,” he said at the fringe, “but this occasional failure of markets does not mean that state control is better.”

Mr. Johnson reserved the bulk of his vitriol for Mrs. May’s proposal, made at her official residence at Chequers, for partial withdrawal from the European Union. “This is not taking back control,” he said, “this is forfeiting control.”

Brussels’ aim? In Boris Johnson’s estimation, it is “to demonstrate above all — to any other country that might even dream of following suit — that you cannot leave the EU without suffering adverse political or economic consequences.”

Mr. Johnson rubbishes as “total fantasy” guarantees that Brexit can be renegotiated at a later date. They “only embolden those who are now calling for a second referendum.” He does seem, though, prepared to delay Brexit until the end of 2020 so as to do an “otherwise redundant and miserable ‘implementation period’” to pursue a “super-Canada trade deal.”

“If we cheat the electorate — and Chequers is a cheat,” Mr. Johnson told the crowd, “we will escalate the sense of mistrust. We will give credence to those who cry betrayal.”

The London Sun reports that “top Tories” are dismissing Mr. Johnson’s appearance as a “sideshow.” They reckon he’ll never become party leader because he is “too much like Donald Trump.” Then again, too, Americans didn’t think Mr. Trump would become president.

BoJo himself leaves Birmingham with a “Brexit means Brexit” reminder. “If we get it wrong, we will be punished,” he warns Tories. “And if we get it right we can have a glorious future.”