British Prime Minister Theresa May’s control over Brexit is slipping away. On Monday, Labour leader Jeremy Corbyn, no doubt hoping to prevent further defections from his party, said he would support a “public vote” on Brexit. He gave no details, but it’s a significant shift in a politician who has never hidden his euroskepticism or opposition to a second referendum.

Whether another vote comes to pass, or Brexit is delayed, now depends very much on if the prime minister can convince lawmakers in her own party to back her deal. It is one of the more curious twists of the Brexit drama that this job — and thus the fate of May and her divorce deal — falls to a lawyer few had heard of a year ago. There is a simple reason for that: Geoffrey Cox may be the only official left who critics of the prime minister’s deal feel they can trust.

It was the attorney general’s damning November legal advice, which the government was forced to publish, that largely motivated Parliament to reject her deal in January. May is now hoping Cox will change his opinion and help her win over enough votes to pass the settlement agreement next month. Failing that, the fate of Brexit looks to be truly out of her hands.

The Cox Factor isn’t to be dismissed. He is possessed of a deep baritone voice, a sharp legal mind and a dramatic courtroom presence; you can feel MPs lean in a little when he speaks. His introductory speech to Theresa May’s Conservative Party conference address in October was a Churchillian barnstormer that left the Tory audience warming to May, a leader they normally barely tolerate. Even if he doesn’t enjoy the same kind of adulation among the party faithful as, say, Boris Johnson, Cox is still a lawyer who can work a jury. And yet it’s a measure of how few cards May has left that she must now rely so heavily on a lawyer’s ability to craft a convincing clause and to dress ambiguity in reassuring legalese.

Cox has been shuttling back and forth to Brussels with a brief to find a way through the impasse on the controversial Irish backstop in the withdrawal agreement. The idea of it, of course, is to avoid the need for any border infrastructure between Northern Ireland the Ireland. Cox had concluded that, as drafted, the clause could keep Britain tied indefinitely to the European Union’s customs union, with Northern Ireland tied even more closely to the bloc’s rules than the rest of the country.

Both “remainers” and “leavers” balked at an agreement that Britain wouldn’t be able to leave on its own accord. Conservative Brexiters, in particular, have objected to long-term membership of the customs union that would make it impossible for Britain to pursue its own trade deals. The Democratic Unionist Party, the small Northern Irish Party that makes up May’s majority, objects to anything that puts Northern Ireland on a different legal footing from the rest of the United Kingdom.

Late last month, Parliament passed an amendment demanding the government seek “alternative arrangements” to the backstop. The EU was aghast that May supported the amendment after arguing the backstop was necessary and non-negotiable. It has refused to concede to demands for a time limit, arguing that doing so would defeat the whole purpose of having a legal guarantee.

It is true, as Cox argues, that if the future trade negotiations break down, then the EU would have no incentive to move things along since the U.K. would be stuck in the backstop. Equally, the EU worries that the U.K. would use any exit clause to weaponize the border issue in the future trade talks. With trust this scarce, no wonder it’s the lawyers who are in charge now.

Ultimately, the EU did agree to reopen a process, but it may be a stretch to call it a negotiation. Cox was meant to address Parliament last week but it was repeatedly postponed. There is feverish talk of a “Cox codicil,” of other kinds of assurances and of trying to find a solution in the terms of the future trade agreement.

The stakes here are enormous. If the EU were to provide sufficient assurances that the U.K. could exit the backstop, there is a good chance May’s deal would win approval in Parliament given that both parties are committed to honoring the result of the 2016 referendum.

But how much fudge will lawmakers swallow? And why would the EU act unless it had a clear sense that Parliament would give its approval? If the EU thinks a delay is inevitable — and averting Brexit a possibility — it may well just decide to leave May to her fate in the shark tank of her own party.

There are reasons for the government to be at least hopeful. First, for those who really want to see Brexit happen, May’s deal, which she has promised to put to a parliamentary vote by March 12, may look like the least bad option. Parliament looks set this week to force her to postpone the exit date rather than accept a no-deal departure if May herself doesn’t preempt the move by offering a tactical delay. Any slippage in the timing brings uncertainty and even the risk that Brexit will not happen.

And Corbyn, who had already made clear he would support a deal involving a permanent customs union, now says Labour will support a second referendum, something the government and many Brexiters want to avoid.

It’s far from clear exactly what he means — and he hasn’t so far submitted an amendment — but the threat ought to concentrate minds on the Conservative benches. Meanwhile, the EU seems to be suggesting a 21-month delay to Britain’s departure, where the outcome of the Brexit process would be entirely uncertain.

In other words, whatever Cox comes up with (or is given) in Brussels may begin to look pretty good to Brexiters if the alternative is no Brexit at all. The government has managed to whittle down a whole array of objections to one issue and the legal opinion of one man. The idea that May’s deal will die a death of a thousand cuts from various sides no longer looks to be a threat. If she clears this one hurdle, she’s through.

And yet we all know by now how big a hurdle it is — even for the likes of Geoffrey Cox.

Based in London,Therese Raphael writes editorials on European politics and economics for Bloomberg Opinion. Previously she was editorial page editor of the Wall Street Journal Europe.

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