LONDON — Westminster remains captivated by the soap opera that is the Conservative Party under Theresa May. But when it comes to the crunch, it might not be the Tories who decide the fate of Brexit.

Their parliamentary rivals, Labour, are approaching an equally momentous fork in the road over the U.K.’s departure from Europe.

Senior decision-makers at the top of Britain’s second party are due to meet in the coming days for talks that "everybody" in the party, according to one senior backbench MP, predicts will presage a major shift in party policy — to back "full participation" in a customs union with the EU.

But an even bigger decision lies ahead for Jeremy Corbyn’s Labour. The biggest political moment of 2018 is likely to be the parliamentary vote on the U.K.’s Brexit withdrawal agreement, expected in the autumn. How Labour’s 259 MPs vote will very likely decide the outcome and the country’s future.

“Labour is clear that we want a deal with the European Union that retains the benefits of the single market and the customs union" — Keir Starmer

At the moment, the party looks like it’s heading only one way: opposing anything other than the softest of departures from Europe.

Its Brexit spokesman Keir Starmer told POLITICO the position was clear: “If the final deal that the government brings back does not meet our six tests and provide for a strong future partnership with the European Union then we will not support it.

“Labour is clear that we want a deal with the European Union that retains the benefits of the single market and the customs union.”

And if May loses the withdrawal agreement vote, what then?

Leading figures in Labour predict either a general election or a second referendum.

Constructive ambiguity

While many of Labour's more vocal MPs want the party to come out in full-throated support of a soft Brexit — close alignment with single market rules and a customs union with the EU — the front bench has made a virtue of constructive ambiguity: ruling few things out, but not actually taking a side.

Labour MPs are waiting for the outcome of an imminent meeting of members of the shadow Cabinet, at which they expect to decide on the type of customs union the party will seek with the EU. Many in Labour had thought the meeting would take place last Friday, but two senior front-bench aides denied that was the case.

Corbyn himself told the BBC's Andrew Marr last Sunday that "a form of customs union" was Labour's intention. A senior Labour official, when asked if the policy shift was coming, said: "A customs union has always been a viable option. Discussions are always taking place about Brexit and the U.K.'s future relationship with the EU."

But at the heart of Labour policy are six tests that the government’s deal would have to meet for Labour to back it. Starmer set them out nearly a year ago, but they hold true.

Does the deal ensure a "collaborative" future relationship with the EU; does it deliver the same benefits as being in the single market and customs union; does it ensure the “fair management” of migration; does it defend workplace, environmental and other rights and protections; does it protect national security and protect cross-border crime-fighting; and is it fair to all regions and nations of the U.K?

The diehard Europhiles in Labour's ranks endorse these goals, but want the party to be more specific when the vote comes. Former Labour Prime Minister Tony Blair has also identified the autumn vote as a moment of vulnerability for Labour.

Predicting that May will come back with a deal on tariff-free trade, and vague commitments on services trade, he said in a January article: "The government will then say it is this deal or no deal and Labour will be left arguing that they would be better negotiators. This isn't credible."

MP Stephen Kinnock, son of the former Labour leader and European Commissioner for Transport Neil Kinnock, believes the party should actively endorse European Economic Area (EEA) membership and a customs partnership with the EU.

“We need to clarify what we mean when we talk about a soft Brexit and that it is the only we thing we can accept when it comes to the meaningful vote in October," said Kinnock, whose stance has won him admirers among the Tory pro-EU faction. The approach has the downside of binding the U.K. to freedom of movement rules, which many of Labour's voters backed Brexit to end.

Nevertheless, backing EEA membership would, Kinnock believes, put the party in a "very strong position politically" and persuade May to bring back a deal that Labour could support, leaving the hard Brexiteers high and dry. It is, Kinnock believes, "the only way she can save her leadership — by isolating the ideologically driven Brexiteers in her parliamentary party."

"It’s increasingly looking impossible to settle this matter in the House of Commons without the people having say on the form of Brexit" — Chuka Umunna

That's one scenario. Another could see May returning from Brussels with a deal that fails to satisfy Labour and pro-EU Tories, risking a government defeat. In that scenario, many, including the general secretary of Labour's biggest union backer, Unite, believe a general election would follow.

"My personal hope and belief is that in later autumn the deal that comes back to parliament will be rejected," Unite leader Len McCluskey told the Resolution Foundation think tank this week. "It will lead to Theresa May having to resign and it will lead to an early general election in 2019. That then becomes a referendum.”

A vote for chaos?

Senior Labour officials are not yet publicly setting out their battle plan for the big Brexit vote and beyond. They do however, according to one senior aide, see one danger on the horizon. Because the government portrays the vote as "deal or no deal," voting it down could look like a vote for political chaos. The government could, the aide said, portray the vote less as a decision on a particular type of future relationship and more a formality to usher in the transition period the government hopes to agree in the next few weeks (and which Labour endorses).

Chuka Umunna, another senior backbencher arguing for Labour to commit to closer EU ties, said that parliament should not be fobbed off with a "fudge" deal.

“I don’t think parliament will allow the government to get away with that," he said. "Also, the EU institutions are clear we will get at least a framework of what the future relationship will look like, so we will at least have a very good idea of the model.”

Umunna was on Friday confirmed as the chair of a new body called the Grassroots Coordinating Group, bringing together the U.K.'s various pro-EU campaigns.

He believes that the party's choice at the final vote goes further than choosing which type of Brexit to endorse. Labour should be "open-minded," according to Umunna, about whether, if voted down by parliament, the Brexit deal should "go to the people."

"The people got a say kicking off the process about whether they wanted to stay in or leave the EU, but they haven’t had a say on the form of Brexit and it’s increasingly looking impossible to settle this matter in the House of Commons without the people having say on the form of Brexit,” he said.