If May decides to cling on even in the face of of a large defeat, her own Conservative MPs could move against her. Party rules mean 48 Tory MPs must write letters of no confidence in order to stage a vote on her leadership. Last month there was an attempt to unseat her, but the rebellion faltered after the number of MPs fell short -- the number of letters submitted was believed to be just under 40.
Even if the rebels could muster enough MPs to trigger a no confidence vote, May could still win it. If she lost, there would be a leadership contest. Many of the mainstream Tory MPs are wary of a weeks-long leadership contest at such a delicate time for the country -- Britain is supposed to be leaving the EU on March 29, and the withdrawal agreement is still up in the air.
Could the opposition force her out?
If Conservative MPs decide to stick with May, there is still the possibility of the opposition parties, led by Labour, forcing her out by holding a vote of confidence in the government. Since the 2017 election, May does not have an overall majority in the Commons, and has to rely on the Northern Irish Democratic Unionist Party (DUP), which has 10 MPs -- a small but potentially significant number.
This week the DUP joined with other opposition parties in voting for a motion which held the government in contempt of parliament, giving a glimpse of how powerful that unofficial coalition can be.
Under the Fixed Term Parliaments Act, the government falls if it loses a confidence vote in the Commons. There are then 14 days for another party to tell the Queen it can form a government. If that happened, Labour leader Jeremy Corbyn could try to form a coalition with smaller parties. This scenario is unlikely, given the DUP, who would be crucial in such a coalition, are unlikely to support a government led by the left-wing Corbyn.
Would there need to be a general election?
If the 14-day deadline passes without a new government being formed, a general election is triggered. May and most Conservative MPs want to avoid am election because they would likely lose even more seats and be unable to form a new government, even in minority. Unless an election is forced on them, it seems an unlikely scenario.
Labour is keen on an election because it made huge gains in 2017 and could end up as the largest party in Parliament, with the power to form a government. However, an election in the current climate would inevitably be dominated by Brexit, and all parties would have to explicitly state how they would handle leaving the EU.
It's all pretty hard to predict.
Can May just renegotiate the deal?
If May loses next Tuesday's vote by a narrower than expected margin -- say, 20 to 30 votes -- she could go back to Brussels and attempt to get a deal that has more support in her own party.
Earlier this whis week MPs voted to have power over what the government should do next if Tuesday's vote is lost -- which could include what a new deal might look like, including a "Norway plus" option where the UK keeps access to the EU single market but no longer abides by some EU rules.
Although EU chiefs have insisted the current withdrawal agreement cannot be changed, in reality it's likely there could be room for maneuver: despite playing hardball for months, Brussels does want a deal with the UK, and to avoid a no deal scenario.
There's an EU summit next weekend, providing a window of opportunity for some quick renegotiation on the terms of withdrawal, particularly around the most contentious aspect -- the backstop which avoids a hard border in Northern Ireland. The current backstop plan has angered the DUP because it would leave Northern Ireland closer to the EU than the rest of the UK. If that changes, the DUP, and many Tory Brexiteers, could be won round and May would win a second vote in Parliament on the new plan.
How likely is a second referendum?
The campaign for a People's Vote, as it is called, is gaining a lot of support among all parties, as well as the former Prime Minister Tony Blair, although May is opposed. There could be an amendment before next Tuesday's vote calling for a second referendum, or, if she loses badly and sees no other way out, the Prime Minister could call one herself. The big question is what would be on the ballot paper: it could be a three-way referendum, with voters choosing between May's deal, a harder Brexit/no deal, or Remain.
Could the UK just crash out without a deal?
This remains a distinct possibility -- particularly because time is running out. Britain could get to a no-deal scenario -- where the country leaves the EU and faces heavy trade tariffs -- under a new Tory leader and Prime Minister, under the different scenarios described above.
With little choice of options, she could go for a no deal to save her own premiership.
Or she could just call off the vote?
All of the above could be avoided, of course, if May calls off next Tuesday's vote. Some Cabinet ministers, and the leader of the backbench group Conservative MPs, want her to delay the vote and go back to Brussels to renegotiate a "better" deal -- that is, one that will get more parliamentary support. However, the EU is unlikely to want to reopen talks until the vote has actually taken place.