London (CNN) As polls close on December 12 and the results of the UK's snap election drizzle in, only one question will matter: is Brexit still happening?

The campaign to stop Brexit received a shot in the arm this week. Three of the UK's smaller parties, the Liberal Democrats, Greens and Plaid Cymru, agreed to a pact in which they would not stand against one another in 60 seats. The aim is to give pro-remain candidates the best chance of winning.

The group, formally known as Unite to Remain, thinks that by consolidating the remain vote, it can take seats from bigger parties who have not fully committed to staying in the European Union.

By contrast, the Brexit vote is divided between the Conservatives and Nigel Farage's Brexit Party. Farage has been criticized for splitting the Brexit vote, leading to accusations, even from former supporters, that he has become the remain alliance's greatest asset. If he does split the vote, there is a good chance that a unified remain vote could win vital seats.

However, remainers should probably keep the champagne on ice. Despite a recent improvement in coordination, the current opinion polls suggest they are still far from winning anything like the numbers needed to revoke Article 50 and stop Brexit.

How Brexit could still happen

Ever since the UK voted to leave in 2016, the main obstacle to any serious decision being taken has been the lack of a majority for any Brexit outcome in the British Parliament.

The aim of the election is to fix that and provide the majority for an answer to this seemingly impossible question. It's worth remembering that all outcomes -- including revoking Article 50 -- would require a majority in Parliament.

As things stand, there are three clear routes to delivering Brexit and two routes to scrapping it.

The easiest, cleanest way to deliver Brexit, is if Prime Minister Boris Johnson wins a majority and gets his Brexit deal through Parliament.

The next simplest route -- at least as far as the legislation goes -- is no deal. For this to happen, the parliamentary deadlock would continue with no clear winner at the election. However, if the balance of power in Parliament lies with more Brexity politicians, then the UK might not request another Brexit delay. This would mean a no-deal Brexit on January 31 and a wild rush to prepare the nation for it. It would take a lot for any leader to actually go through with this, but if that's where the parliamentary maths ends up, it's a real possibility.

Finally, there's a Labour Brexit. If Labour ends up in power -- either in a coalition or with its own majority -- then it pledges to renegotiate a new Brexit deal. It would then hold a public referendum between the deal and remaining in the EU. The party would be neutral during this campaign.

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How Brexit could be canceled

The outcomes that lead to remaining in the bloc are slightly more complicated.

In terms of pure numbers, the most likely way for Brexit not to happen is that Labour takes power in some kind of coalition or arrangement with the remain alliance. The remain alliance could then back Labour's second referendum in Parliament and back remain in that vote. Where the parties might disagree is on Labour's promise to negotiate a new Brexit deal beforehand -- it doesn't look very remainish to enable a party that's negotiating a Brexit deal.

The other, much less likely, outcome is that a majority exists for straight remain.

It's certainly true that if you took the Members of Parliament from Labour, the remain alliance and Scottish Nationalist Party, the vast majority in that group would have voted to remain in 2016.

This statistical reality is not lost on pro-EU campaigners, who have set up websites encouraging remainers to vote tactically. The idea is simple: you search online where you live and it tells you which pro-remain candidate is most likely to win in your area.

The problem is, it's hard to predict what actually happens when MPs return to Westminster after the election. Party whips exist and Labour remain MPs will be under enormous pressure to obey the whip.

These tactical-voting websites seem to recommend voting for the Labour candidate in a lot of seats, despite Labour's policy being to negotiate a Brexit deal.

Why? At this election, the vast majority of contests will be between Labour and Conservative candidates. As Rob Ford, professor in politics at the University of Manchester, points out, "If these websites only backed continuity remain candidates, they would be hurting Labour in seats that could easily fall to Conservatives. Remain voters have to make a choice: do they want a Labour MP that gives them half a loaf of bread, or a Conservative who gives them nothing?"

This might be true. But it's also true that party whips will be looking to asset their authority after the election. Ford explains, "If remain voters back Labour candidates, then they leave themselves open to party whipping. They could elect an MP who ultimately walks through the lobby for leave."

And a referendum?

There are still miles left to travel on the road to delivering Brexit. But the journey to blocking it looks even longer. The most likely way for that to happen is a Labour government giving the decision back to the public.

So, after what's already been a deeply nasty election campaign, we would have an even more divisive referendum. Brexiteers would claim it was a choice between remain or remain. The remain factions, famously full of egomaniacs, would squabble over who leads the campaign.

And once it was all said and done, the referendum victor would have to return to Parliament. And in this scenario, you'll have noticed, there is still no majority for anything.

So, can Brexit be stopped? Yes. But it's even harder than getting Brexit done. Which, it goes without saying, is no walk in the park.