Barring phenomenal bad luck on his part, Boris Johnson is the Conservative MP most likely to succeed as party leader and British prime minister. He is the favorite MP of declared parliamentary colleagues who will choose two leadership candidates to put before the membership. And among Conservative rank-and-file, BoJo is far and away their favorite Tory in Parliament.

A few short months ago, most Brexiteers would swoon at the prospect. During the 2016 referendum, BoJo was the most colorful proponent for British independence from the European Union . When “Remain” premier David Cameron resigned after the decisive vote to leave, expectations were dashed when Mr. Johnson decided not to “climb the greasy pole” to the top job.

Instead, Britain got Theresa May and the rest is a sad litany of Brexit betrayal, mendacious mandarins, and pitiful parliamentary posturing. Now it is Mrs. May’s time to leave 10 Downing street. Mr. Johnson will not let it pass him by again. As Conservative MP Andrew Bridgen puts it, “The only person who could stop Boris would be Boris.”

Therein lies the rub. Is Mr. Johnson the leader to make Britain’s independence from the European Union a reality, fulfilling the long-term promise of Brexit? Conservative parliamentarians are among his severest critics. Nor need we trawl his record as London mayor or Foreign secretary for missteps. The last six months will do.

His tacit support of Mrs. May’s Withdrawal agreement (for fear of something worse) is the most conspicuous mark against him. Turning down an invitation to meet President Trump during his state visit another. Ruling out working with Nigel Farage and his nascent Brexit party yet another inexplicable mistake.

The Brexit Party, after all, is the singular achievement of the year to date and marks the most significant step toward Brexit since that historic referendum vote to exit. Mr. Farage, though he is willing to work with anyone committed to Brexit, is sceptical of Mr. Johnson’s sincerity. (Several “Remain” Conservatives are rumored to be backing BoJo on the bet that he would prove to be “malleable.”)

Lest there be any mistake, your Diarist likes Mr. Johnson’s élan and joie de vivre. BoJo’s cheerful insouciance is just what the doctor ordered for our collective Brexit blues. Brexiteers, however, are understandably battle-weary from false starts and “Brexit means Brexit” promises unkept. Once bitten, twice shy.

So apart from Mr. Johnson’s avowal to leave the EU by the stated end-of-October deadline, do Brexiteers look in vain for added assurance?

Thankfully, no.

The Conservative party doesn’t want to be annihilated at the polls — the unquestioned result were Brexit to be further postponed. Before, Tories may have taken comfort that no other party had Brexit street cred; now, Mr. Farage and his Brexit party champing at the bit strike fear into the heart of Conservatives.

Moreover, Mr. Johnson has attracted to his camp such noteworthy Brexiteers as Jacob Rees-Mogg. Others backing Boris — James Delingpole calls them “Spartans” — never wavered in their Brexit faith and stood firm against Mrs. May’s Withdrawal Agreement. Parliamentarians such as Steve Baker, Crispin Blunt, Peter Bone, Sir William Cash, Mark Francois, Priti Patel, and Sir John Redwood are prominent among this number.

Worried Brexiteers can take heart these Spartans will not backtrack on British independence. Brexit is their Thermopylae.

After Boris Johnson’s campaign launch, Mr. Delingpole, no Johnson acolyte, voiced a note of optimism. “What the party needs — if it is to survive — is a leader prepared to take the fight to the enemy and not apologise for speaking freely and honestly or for advocating Conservative principles,” he writes. “Boris — who evidently has a lot of goodwill and momentum behind him — could well turn out to be that man.”

Mr. MacLean, who maintains the weblog The Organic Tory, covers Brexit for The New York Sun.