LONDON — Theresa May’s Brexit deal looks unlikely to win the backing of the U.K. parliament.

Opposition parties are against the deal as it currently stands; dozens of Conservative MPs (more than 80 by some counts) — both Leavers and Remainers — also oppose it; the Democratic Unionist Party that props up Theresa May's government is against.

In favor: the rest of the Conservatives' 315 MPs and, possibly, a low number of Labour MPs fiercely committed to delivering Brexit for Leave voters in their constituencies. Not enough to win, as May’s parliamentary enforcer, Chief Whip Julian Smith, reportedly told the prime minister this week.

While numbers in the pro-deal bloc could still grow as MPs digest the text of the Political Declaration on the U.K.'s future relationship that May agreed with the Commission Thursday, they could just as easily shrink if MPs see things they don’t like in the final document.

No timetable is set for the U.K. parliament's vote but most expect an early December showdown. If (when?) MPs reject May’s deal, then what?

Here’s how things might go, according to some key players who will shape the process:

The technicalities

As well as voting, members of both Houses of Parliament also have opportunities to attach amendments to motions or bills, some of which can change the meaning of the vote very significantly.

Speaker of the House of Commons John Bercow, the highest authority in the House on procedural matters, has said the government's motion — effectively asking MPs to accept or reject May's deal — will be amendable. It remains uncertain whether these amendments could be made before the main vote. This will be decided upon following a debate and decision by the House of Commons itself.

This matters because if MPs get a chance to vote on an alternative before voting on the main question, Labour could push an amendment (for example, by demanding a permanent customs union, as is their policy) that changes the complexion of the deal before it is even voted on.

Pro-EU Conservative MP and former Attorney General Dominic Grieve told POLITICO he expects this won’t happen and MPs will first have to vote on the main question — to accept or reject May’s deal — before they get into the tricky business of amendments. Other MPs disagree.

Even if parliament does not get to amend the government’s "deal or no deal" motion, Labour’s Shadow Brexit Secretary Keir Starmer told the party’s MPs on Monday he expects legislation will be required to keep Britain functioning in the event the U.K. leaves the European Union without a deal — legislation that could be amended to bind the government to a new course of action.

“There will be opportunities to make the majority [in parliament] against no deal heard. Motions will be tabled, amendments will be pressed and a no-confidence vote [that could potentially bring down the government] can be triggered," he said.

Grieve agrees. “It would certainly be possible for parliament to express its view [if it faced leaving the EU without a deal],” he said. “It could do it on a backbench day debate or for that matter on an opposition day debate.”

Blocking no-deal Brexit

One key thing to note in this process is the Labour Party holds most of the cards.

It is they, with their 257 MPs, who will make or break any effort by parliament to force the government down one or other path.

Although May has portrayed a vote against her deal as a vote for no deal, Starmer told the party’s MPs on Monday evening that the first priority is to prevent the U.K. crashing out of the EU.

“No deal is not an option. Labour will not countenance no deal — and nor would many of the prime minister’s own MPs," he said.

No wonder some Cabinet ministers are beginning to drop the "May’s deal or no deal" rhetoric.

He will be joined in these efforts by other opposition parties — the Scottish National Party, with 35 MPs, the centrist Liberal Democrats with 12, Welsh nationalist party Plaid Cymru with four and the one Green MP.

SNP Leader and Scottish First Minister Nicola Sturgeon has been in Westminster this in week attempting to forge a united front between the opposition parties. They cannot agree on a single route forward, but they do agree on blocking no deal.

A significant number of Conservative MPs are also dead set against such an outcome.

No wonder some Cabinet ministers are beginning to drop the "May’s deal or no deal" rhetoric. Work and Pensions Secretary Amber Rudd told BBC Radio 4’s Today program on Wednesday: “Parliament, the House of Commons, will stop no deal. There isn’t a majority in the House of Commons for that to take place.” Chancellor Philip Hammond, however, told ITV's Robert Peston on Wednesday: "It could be no deal, but it could be no Brexit, we just don’t know." He added that if MPs reject the deal, the U.K. enters a "politically chaotic situation."

Purely on the question of the majority in parliament against no deal, Rudd is right.

Rethink under pressure

Global market reaction to a rejection of May’s deal would likely be dramatic, with a potentially sharp fall in the value of the pound.

Under this kind of pressure MPs may feel inclined to amend the deal on the table rather than reach for more chaotic options, such as a general election or a second referendum. Former Treasury official Rupert Harrison has dubbed this the "TARP" scenario — a reference to the U.S. bank bailout plan in 2008 that was initially rejected by Congress, causing a market shock that persuaded lawmakers to back an amended version at a second attempt.

Such a scenario opens the possibility of parliament seeking to force the government into a renegotiation.

Depending on which of the many possible amendments secure majority support, May could be sent back to Brussels seeking the kind of softer Brexit demanded by opposition parties and some Tory MPs, like Nick Boles, who argues for a Norway-style model. Alternatively, a successful amendment could require the prime minister to pursue a Brexit without aspects of her deal most hated by Brexiteers (for example, the lack of a unilateral break clause on the Northern Ireland backstop).

How the EU would respond to such renegotiation requests is difficult to predict — and the countdown clock will not have stopped ticking.

Cabinet minister Andrea Leadsom, who as leader of the House of Commons organizes government business in parliament, told MPs on Thursday they "should not be under any illusion that the EU would be ready to start all over again and negotiate a different deal." Pushing the government's line that this is a straight deal or no deal choice, she added: "It is very important that we do end up with a straightforward approval of the deal.”

Certainly, the likelihood is that EU member countries and the European Commission, having reached a delicate conclusion to negotiations once already, will be reluctant to reopen the file.

But if May were to return from such a renegotiation with much the same deal, with the pound crashing and opinion polls suggesting the public is crying out for a deal one way or another — perhaps the views of those currently opposed would change and MPs would back the prime minister's Brexit agreement on the second attempt.

More time

A renegotiation may require more time. The legally enshrined Brexit date is already just four months away. One means by which parliament could block (or at least delay) a no-deal exit is by demanding May ask for the negotiating period to be extended.

Grieve said such an extension would be “absolutely essential” if May’s deal is voted down, to allow time for a new course to be set.

Extension would require a U-turn from May, who has ruled it out. It would also need unanimous approval from EU27 countries.

Charles Grant of the Centre for European Reform think tank said he thinks the upper limit of any extension could be “mid-May” because of the upcoming European Parliament election, but in extremis the EU could be flexible.

“Britain’s seats in the European Parliament have already been reallocated and it would be legally complicated to keep the U.K. in the EU beyond the elections,” Grant wrote in a blog for the CER. “But if the EU really wanted to prolong British membership by several months, there could be ways around the European Parliament problem; for example, the U.K. could appoint MPs as MEPs on an interim basis.”

Another election

Importantly, Labour’s preferred outcome following a rejection of May’s deal is a general election.

Speaking to MPs on Monday, Starmer listed a vote of no confidence in the government as one option to prevent no deal. If the government lost such a vote by a simple majority, and no new administration could be formed by other parties in 14 days, an election would have to take place.

However, there is a big block to this route: Conservative MPs.

While many of them may be preparing to vote against May’s deal, few are willing to bring down their own government and trigger a chain of events that could end with Labour leader Jeremy Corbyn as prime minister — and some of them losing their jobs. Tory MP Damian Collins told the Today program on Wednesday he might be able to countenance an election as a way out if there was an impasse in parliament, but Grieve said he is deeply skeptical.

“Everything’s got to be on the table but what’s important now is that we focus on where we can build majorities" — Nicola Sturgeon

“Labour may wish to try but I don’t think they’re going to be successful," he said. "I simply don’t see where their majority for that is going to come from."

Sturgeon also hinted she is similarly skeptical.

Asked about the option after meeting Corbyn on Tuesday, she said: “Everything’s got to be on the table but what’s important now is that we focus on where we can build majorities. We can all table different things but if they get voted down because we haven’t done the work to build the majority behind them it’s not going to take us very far.”

Second referendum

Grieve’s preferred option, and one viewed favorably by Sturgeon and other smaller opposition parties, is a second referendum.

The former attorney general believes 10 to 15 of his Conservative colleagues back such a plan (several are already publicly supporting the option) and that a larger group could be persuaded if parliament is in a state of deadlock after rejecting May’s deal.

He said May’s deal looks like “an extraordinarily third-rate outcome which leaves us significantly disadvantaged compared to remaining in the EU.”

“I’m not prepared to just let it go through on the basis that I’m going to make the decision and tell the public this is what we’ve decided — I won’t do that. I want a referendum on it,” he said.

One obstacle is agreeing the question.

Grieve's preferred referendum question would be a choice between May’s deal and remaining in the EU. Another option would be to put leaving without a deal on the ballot paper as a third option, something favored by another Tory advocate of a second referendum, Justine Greening.

Ultimately, of course, it does not matter how many Conservative MPs might back a second referendum if Labour do not.

The party has not ruled it out if they fail to force an election, and both leader Jeremy Corbyn and Shadow Chancellor John McDonnell have said in recent days the option remains on the table.

Sturgeon believes their position could evolve. “The option that would stop Brexit is another vote and Labour’s official position is not for that at the moment but hopefully over the course of the next few weeks as these discussions develop we’ll see movement on all sorts of positions,” she told reporters in Westminster on Tuesday.

Tom McTague contributed reporting to this article.