Boris Johnson will fire up the charm in an attempt to push his Brexit deal through the U.K. parliament — but crushing failure followed by an election could be no bad thing.

The British prime minister will embark on a final full day of domestic diplomacy Friday as he tries to cajole opponents to back him in a crunch showdown on Saturday when the House of Commons will vote on his deal.

Johnson already started greasing the wheels on Thursday after he clinched a surprise agreement with Brussels and jetted to the European Council summit in Brussels to get it rubber-stamped by EU leaders. “He’s been making calls today from here,” a senior government figure told reporters. “He is going to be talking to people tomorrow throughout the day.”

Saturday's vote is the crucial next stage in the Brexit process, but an expected defeat in the House of Commons coupled with the win from Brussels could hand him a tailor-made election campaign pitch to take to the public: Parliament blocked my deal and refused to deliver Brexit, so give me more MPs to get it done.

The deal came suddenly during mid-morning on Thursday, when the prime minister held a call in Downing Street with European Commission President Jean-Claude Juncker to sign off the final sticking point: an exemption for Northern Ireland from VAT rules on sanitary products, meaning Britain can finally scrap the so-called “tampon tax.”

After a quick check with senior Cabinet colleagues, Johnson called Juncker back and confirmed the deal was on. He dashed out of Westminster and to City Airport, where he boarded his trusty RAF Voyager for Brussels.

Johnson was jubilant at the European Council summit, where EU leaders took less than three hours to sign off his new agreement.

For a Downing Street so often portrayed by the opposition — and indeed by resigning Cabinet ministers like Amber Rudd — as insincere about a clinching a pact, the culmination of negotiations brought a certain schadenfreude.

“No one can now sling mud about him not being serious about getting a deal,” a Downing Street official said.

Bounding into a press conference on Thursday evening, Johnson told reporters: “It will be a very exciting period now to get to the positive side of [the Brexit] project. The extraction having been done, the building now begins. And I'm very confident that when my colleagues in parliament study this agreement, that they will want to vote for it on Saturday, and then in succeeding days.”

First rule of politics

But his most immediate colleagues — Northern Ireland's Democratic Unionist Party who agreed to support the Conservative government after the last election — dealt him a major blow on Thursday morning when they confirmed their 10 MPs could not support his plans. The Brexit deal entails checks on some goods being shipped from Great Britain to Northern Ireland, breaching the “blood red line” of no trade border down the Irish Sea that was set by DUP leader Arlene Foster.

“These proposals are not, in our view, beneficial to the economic well-being of Northern Ireland and they undermine the integrity of the Union,” the party said in an angry statement. A spokesman confirmed the DUP will not just abstain but will vote against the deal on Saturday.

The decision throws the parliamentary arithmetic into chaos for Johnson. The DUP was seen by Downing Street as the route to unlocking 28 of the most pro-Brexit Tories in the so-called European Research Group, who refused to back the deal Theresa May agreed with Brussels. The ERG was then seen as the route to unlocking around 20 Labour MPs who represent Leave seats and want Brexit done.

Many of the key figures in those factions kept quiet on Thursday — or at least appeared to keep their options open — in an encouraging sign for the prime minister. The ERG will meet Saturday morning to discuss their position. An aide to a Labour MP said that more of their group were thinking about backing the deal than was previously expected. Nineteen wrote to Juncker and European Council President Donald Tusk earlier this month, pledging their support for a Brexit pact.

UK NATIONAL PARLIAMENT ELECTION POLL OF POLLS

For more polling data from across Europe visit POLITICO Poll of Polls.

Johnson made them a direct and public appeal during his press conference, hinting that sweeteners they have been demanding — on environmental protections and workers’ rights — could be within their grasp if they back him on Saturday.

He said the Political Declaration on the future relationship with the EU contains “important provisions and commitments that this country gladly makes about our determination to maintain the highest possible standards, both with the environment and the social protections.” He added: “We make those commitments gladly and they are entirely right for us to do.”

Party political broadcast

But Johnson also made clear during his press conference that he is ready for plan B if the numbers in parliament fail to materialize.

“This is our chance in the U.K., as democrats, to get Brexit done, and come out on October 31,” he said. “This is our chance to focus on our priorities, the people's priorities, the NHS, putting 20,000 police on the streets, lifting up funding of education across the country, the biggest expansion of the living wage.”

The manifesto list was a reminder that the prime minister wants a general election soon. If he loses the vote in the Commons on Saturday, and the parliamentary arithmetic makes it likely he will, he will be forced to trigger a Brexit delay and will likely launch another push for an election.

Opposition parties who insisted they would back a poll once they had legislated to prevent a no-deal Brexit will come under intense pressure to back it.

If an election does come about, the Conservatives could fight it on a ticket of "Get Brexit done, with this deal," but Downing Street declined on Thursday to reveal any planned messaging.

“We have always backed an election. All I would say is the thing this week is to get Brexit done. The deal is here,” a No. 10 official said.

Such an election stance would leave the Tories open to attack from the Brexit Party, led by Nigel Farage, who was quick to dismiss Johnson's deal. It was, he tweeted, "not Brexit" because of what he described as "the commitment to regulatory alignment," a reference to the pledge to match EU rules for some goods in Northern Ireland.

A Brexit Party official confirmed that if the Conservatives adopted the new deal as policy, Farage’s offer of a Conservative/Brexit Party pact at the next election (admittedly, something senior Tories have never publicly entertained) would be dead.

Message discipline

Downing Street remains confident the clarity of their message could still win the day in an election.

With opposition parties now pulling in many different directions — and indeed, in the case of Labour, internally divided on what course to take — there is hope a deal which could turn Brexit theory into Brexit fact will only help ram home the “Get Brexit done” messaging, and sideline the problem of the DUP in the process.

“It gives us all the more reason to have one very clear message,” the first official said. “The polling reflects that the clearer we are in our mission the more likely we are to win the support of the public.”

Undeniably, Johnson and his government are at the mercy of events, and Downing Street officials privately acknowledge that many roads could now open up. The deal could pass cleanly on Saturday, or perhaps with an amendment demanding it be put to a second referendum. MPs could finally back an election, or stalemate could continue, with no clear way forward receiving majority support.

The precise strategy in each scenario is unknown to all but a few around the prime minister, and to some extent even they will have to react to unfolding events.

The senior official said: “It’s so unclear as to whether there would be a majority for anything. We’ll have to wait and see.”

Nevertheless, Johnson may find he has a few more hands to play.

This article is part of POLITICO Pro’s premium Brexit service for professionals: Brexit Pro. To test our our expert policy coverage of the implications and next steps per industry, email pro@politico.eu for a complimentary trial.