It’s not that British Prime Minister Theresa May will lose the upcoming vote to have Parliament sign off on the Brexit deal she has painted as an all-or-nothing proposition.

That result, according to Westminster watchers, is a foregone conclusion.

What will define her political fate and the future of her mishap-plagued leadership will be just how badly she loses.

After surviving the loss of her majority at last year’s election, and hot on the heels of a series of embarrassing high-profile Cabinet defections, the parliamentary vote is shaping up as what will prove to be the first shot in Mrs May’s desperate last stand.

A narrow loss would cool the ambitions of Conservative Party rivals who ache to  replace her in Number 10 Downing Street, the official prime ministerial residence.

A whopping defeat, on the other hand, would be a declaration that it is open season on the embattled leader, with calls for her resignation inevitably coming almost from the moment the votes are tallied.

Britain would be doing itself enormous damage if it quits the EU, according this Remain supporter. Photo: AP/Frank Augstein

Mrs May has clung to power since June 2017, when a clumsy and gaffe-prone campaign saw only 42 per cent of voters back her Conservatives.

Reeling from a near defeat the polls didn’t predict when the campaign was launched, she was forced into an uneasy partnership with Northern Ireland’s Democratic Unionist Party (DUP), whose 10 members have since indicated they will reject the Brexit deal.

According to the best estimates, based on declared opponents and likely ones, Mrs May will fall short of her goal, with some 60 per cent of members in the 650-seat House of Commons expected to vote ‘no’ when push comes to shove on the floor of the House on Tuesday (local time).

Theresa May admires the Christmas tree outside No. 10, but it won’t be Santa deciding if her Brexit pact is naughty or nice. Photo: EPA/Andy Rain

Mrs May has insisted that the choice on offer is her deal or no-deal, but pro-EU lawmakers reject that “false dichotomy” and want Parliament to consider other options.

Eurosceptic Conservatives, on the other hand, accuse Mrs May of negotiating a pact that imposes all the disadvantages of the existing Brexit arrangement while bestowing no significant benefits and doing so at enormous cost.

She insists the deal honours “the wishes of the British people” after a 52-per-cent majority voted to leave the EU in the 2016 Brexit referendum.

The DUP opposes the deal on the grounds of a temporary backstop measure to guarantee an open Irish border after Brexit. The backstop could place Northern Ireland under slightly different trading arrangements from the rest of the United Kingdom.

What happens if Parliament, as expected, rejects Mrs May’s deal could depend on “how badly, and in what way, she loses”, said John Curtice, a political scientist at the University of Strathclyde.

A narrow defeat of 20 votes or less “would almost be a moral victory”, he noted, allowing her to tweak the deal before going back to Parliament for a second shot.

The result will depend on whether there are “more Labour supporters of the government than there are Conservative opponents of the government”, Conservative right-winger Jacob Rees-Mogg, who leads a group of several dozen eurosceptic lawmakers, told the political news website Conservative Home.

Labour’s 257 members of Parliament will oppose Ms May’s deal, Left-wing leader Jeremy Corbyn said, although a handful of Labour rebels are expected to break ranks and back the Prime Minister.

“It [May’s deal] is a worst-of-all-worlds deal that works for nobody, whether they voted leave or remain,” Mr Corbyn wrote in Friday’s Guardian.

Labour leader Jeremy Corbyn has his eye on an election. Photo: Getty

“We are working with MPs and parties across the House of Commons not only to ensure it is rejected, but also to prevent any possibility of a no-deal outcome,” said Mr Corbyn, who wants Britain to stay in a customs union and keep “a new and strong relationship with the single market”.

Parliament watchers dismiss that claim as hollow rhetoric intended to disguise a far more immediate goal.

“Labour’s view at the moment seems very much more strongly centred around the possibility of collapsing the government, rather than saving a deal,” said Simon Usherwood, a Brexit specialist and political analyst at the University of Surrey.

A growing number of Labour and Conservative lawmakers are backing the People’s Vote campaign for a second referendum, which Mrs May has ruled out.

Mr Corbyn has suggested such a plebiscite would only be a last resort for Labour, which hopes to force an election instead.

Where will the chips fall?

Here are some of the possible scenarios.

Plain sailing: On Tuesday, Parliament approves the withdrawal agreement and further negotiations already endorsed in principle by EU member states.

From the outset, a victory for Mrs May looked unlikely. Since Parliament began debating the Brexit package, the odds have fallen further.

Rough waters: Parliament narrowly rejects the deal. Mrs May returns to Brussels to seek amendments in order to put the agreement to a second vote.

As analysts note, this approach would most likely work if her defeat in Parliament is not too large.

“If her deal is defeated by a big margin – say by 30 to 40 votes – she may no longer be in control of events,” warned Mujtaba Rahman, of think tank Eurasia Group.

Furthermore, the EU side has made clear that the fundamentals of the withdrawal deal are not up for renegotiation, so any changes would likely be cosmetic.

Mutiny: If Parliament overwhelmingly rejects May’s Brexit package, this would put her leadership into question, potentially bringing Britain closer to a general election. It would also strengthen the resolve of those calling for a second referendum on Brexit.

In this case, any minor changes to the deal are unlikely to be enough to placate its opponents.

Titanic, meet iceberg: If the British Parliament does not accept the withdrawal deal and it cannot be ratified before March 29, this would put Britain on track for an unregulated Brexit.

EU rules would cease to apply in Britain as of March 30 and the country would drop out of shared arrangements – such as common air traffic rules or trade deals with third countries.

All of this would lead to uncertainty for British citizens living in the EU and vice versa. On the upside for London, it would be released from many remaining financial obligations to Brussels.

Back to port? On Monday, the European Union’s top court could decide Britain has the right to change its mind about Brexit, emboldening those calling for a second referendum and the entire process to begin all over again.

-with AAP