Will British politicians OK a Brexit bill that few are happy with?

Members of Parliament in Britain began five days of momentous debate Tuesday on the terms of withdrawal from the European Union. The parliamentary debate and subsequent vote on the Brexit agreement negotiated by Prime Minister Theresa May could be a breaking point for her minority government. She faces an uphill battle to convince her ruling Conservative Party to back her amid a string of cabinet resignations and an abortive leadership challenge.

In 2016, 52 percent of United Kingdom voters in a national referendum chose to leave the EU. Since then, the painful process of untangling more than four decades of legal and economic integration with Britain’s closest trading partners has frustrated many “Leave” voters. “Remainers” have argued that it’s not too late to reconsider a policy that over time is likely to lower living standards in Europe’s second largest economy, compared with staying in the EU.

WHAT WILL MPS BE CONSIDERING AND WHY IS IT SO MOMENTOUS?

Parliament is being asked to approve Ms. May’s withdrawal deal and a framework agreement for future relations between Britain and the EU. The 585-page withdrawal deal has already been approved by leaders of the 27 other EU members. It still requires the assent of the European Parliament, in which Britain is currently represented.

British MPs are considering the most significant change to Britain’s trade, security, and foreign relations since it entered the European Economic Community, as it was then known, in 1973. And they must do so under huge pressure from both sides of an emotive issue that has divided the nation and the two largest political parties.

The withdrawal deal sets out in detail the terms of Brexit, including the $45 billion owed by Britain to the EU, the rights of EU citizens living in the UK and vice versa, and the status of the land border between Ireland, an EU member, and Northern Ireland, which is part of the UK. It also creates a 21-month transition period after Britain leaves in March that should allow time to agree on the parameters of a future relationship.

The much shorter framework agreement is less contentious, since it is a broad political declaration by both sides. “It sets a direction of travel but is in no sense binding,” says Anand Menon, director of the UK in a Changing Europe Initiative at King’s College London.

What is highly contentious is an arrangement for keeping the intra-Ireland border open in the event that negotiators fail to agree on a permanent trade pact. That arrangement, known as a “backstop,” would keep Britain in the EU’s customs union. It would not be the clean break that pro-Brexit MPs seek and would handicap Britain in forging its own trade deals.

WHAT IS THE TIMETABLE FOR APPROVING THE BREXIT DEAL?

The vote will be held in the House of Commons on Dec. 11 at the end of the fifth day of debate. MPs will first vote on several amendments to the motion to approve the withdrawal agreement and framework for future relations. These amendments could prove decisive in the event of a defeat for May, which many analysts see as a likely outcome.

One amendment that has cross-party support would give MPs the right to block a “no-deal” Brexit, under which Britain leaves without any agreement in place. This amendment passed on Tuesday. Other proposed amendments go further in pushing for a second referendum in the event of a failed vote. May’s government insists that the withdrawal agreement is the only option on the table and that its rejection would mean a no-deal exit.

Even before the debate began there was uproar in Parliament over the government’s refusal to publish in full the legal advice of Attorney General Geoffrey Cox over the Brexit deal. Mr. Cox told MPs on Monday that it wasn’t in the public interest to release it. In response, MPs voted to hold May’s government in contempt, the first time ever in British parliamentary history.

WHICH FACTIONS ARE LIKELY TO OPPOSE THE DEAL?

The opposition Labour Party says it will vote against the withdrawal agreement because it doesn’t go far enough to protect British workers and companies from the cost of leaving the EU. Behind this stance is a calculation that May’s government could fall and that Labour has a good chance of winning an early election. Some Labour MPs support May’s deal as a better option than crashing out of the EU.

May’s biggest challenge is convincing her own party to approve the deal. Dozens of pro-Brexit MPs have already vowed to vote it down. The most common objection is that the backstop would trap Britain in a position of being subject to EU rules without having a say over them. A minority of Conservative MPs want to defeat the deal as a path to stopping Brexit.

Among the smaller parties, the Democratic Unionist Party, which is allied with the Conservatives, is likely to abstain or vote against the deal. Nor can May count on votes from the Scottish National Party or the Liberal Party. That puts the onus on Conservative whips to cajole their members to support May’s deal as the only realistic path to an orderly exit.

WHAT HAPPENS IF PARLIAMENT VOTES IT DOWN?

The government could try again. One argument is that a sharp fall in Britain’s currency and stock market over the risk of a chaotic Brexit would force Conservative MPs to reconsider, just as the United States Congress blinked in 2008 when the Bush administration pressed for emergency bailout funds. But the parallel is not exact and the market reaction may be muted.

Pro-Brexit MPs have urged May to go back to Brussels and wring more concessions to make the withdrawal agreement palatable to Parliament. Both sides insist that this is the final deal, take it or leave it. But the EU has shown flexibility in handling past crises.

“If this deal is rejected the likelihood is we’d have a slimmed down version without the backstop,” says Daniel Hannan, a Conservative member of the European Parliament and a founder of the Vote Leave campaign in 2016.

Much depends on the margin of any defeat. Winning back 15 or 20 Conservative MPs is feasible. But a major defection by May’s party would weaken her position as leader and could bring down her entire government if rebel MPs back a no-confidence motion. Labour will be seeking to force an election if the Conservatives are unable to form a minority government.

IS IT POSSIBLE TO REVERSE BREXIT AT THIS LATE STAGE?

Yes. Campaigners for a second referendum argue that if Parliament is deadlocked then voters should decide on whether or not to accept the Brexit deal. That would leave open the door to staying in the EU. Critics say this would be undemocratic and deeply divisive.

Britain is due to leave the EU on March 29. This could be extended so that Britain can reach a political consensus on what to do. The British government has argued that stopping the clock is impossible without the approval of other EU members. But a senior adviser to the European Court of Justice said Tuesday that the UK could decide unilaterally to halt the Brexit process.

Any extension to Brexit could be protracted, since the European Parliament will hold elections in May, and the European Commission’s leadership is due to change next fall. Another year of stop-go talks is likely to swell the ranks of Britons who are resigned to a BINO (Brexit In Name Only) that fails to fully disengage from the EU.