British Prime Minister Boris Johnson lost his majority in the House of Commons on Tuesday with the defection of a Conservative member of Parliament to the opposition Liberal Democrats. That sets up an unpredictable chain of events that could derail Johnson’s promised Brexit strategy and throw the nation into a serious constitutional crisis.

Johnson’s loss of his majority almost surely means that his government will lose the vote scheduled for Tuesday evening on a bill designed to prevent a so-called no-deal Brexit. Since Johnson has stated he will not accept that bill if passed, this forces the opposition’s hand. It has two likely options: Sack him and replace him, or vote in favor of new elections in mid-October.

The first option is perilous because the opposition is fractured. The Labour Party has the largest number of seats in the opposition, but is far from a majority on its own. It needs the support of six other parties, independents and some Conservatives who are defecting from the government to form a narrow majority. Many of these MPs recoil at the thought of making the far-left Labour leader, Jeremy Corbyn, prime minister even for a short period of time. Thus, we may not see an opposition-backed prime minister unless Corbyn is willing to back someone else for the top job — something he has not indicated he would accept.

New elections are a big risk for the opposition, however, because polls show Johnson’s Conservatives would likely win them. Britain’s Parliament is elected in single-member districts where a candidate need not get a majority to prevail. This “first-past-the-post” system means that Tories could easily win a majority of seats with only 35 percent of the national vote. Given that more than 400 of the United Kingdom’s 650 constituencies voted for Brexit in the 2016 referendum, Johnson has the upper hand.

Opposition parties could agree to an electoral pact to oppose Conservatives, but that’s easier said than done. The Liberal Democrats and the Greens are staunchly pro-Remain; they do not want Britain to leave the European Union at all and have been pushing Corbyn to unambiguously support a second referendum that could cancel Brexit entirely. Corbyn is loath to embrace that option, however, as his party’s voters are divided on Brexit, with many working-class supporters in favor of it. He is also a lifelong euroskeptic and does not personally back staying in the E.U. as is. Without a pact, Johnson would surely triumph unless large numbers of Brexit backers plunked down for Nigel Farage’s Brexit Party. So far polls show Brexit-backing voters flowing toward the Tories and away from Farage.

Johnson himself cannot call for new elections even if he no longer controls the Commons. The Fixed-Term Parliaments Act allows for new elections only if two-thirds of the House of Commons agree or if a majority backs a motion of no confidence but does not install a new government within 14 days. If the opposition can’t agree on a government and doesn’t want elections, Johnson remains a neutered prime minister.

This means the parliamentary opposition could try a novel strategy: Keep the government, and tie its hands. This approach might pass the anti-no-deal bill but refuse to vote Johnson out of office. It would contend that under the British constitution, the government would be required to enact the will of the parliamentary majority. But Britain has never had a situation such as this in its democratic history, one where a parliamentary majority refused to take the reins of government itself. Simply trying to do this is uncharted constitutional territory.

This would give Johnson an option to force its hand: Advise Queen Elizabeth II to withhold royal assent from the anti-no-deal bill. The monarch retains the power of an absolute veto, but it has not been used since the early 18th century. Convention requires her to give assent to a bill backed by Parliament, but convention also dictates that she follow the advice of her prime minister. Since the prime minister has always controlled Parliament, she has thus always assented to the bill. But what will she do if her ministers advise a veto in the face of a parliamentary majority that refuses to take power?

Parliamentary democracy requires the existence of a government that can command the floor of the House at all times. If the opposition remains loyal to this fundamental and core principle, then Britain will likely face new elections in October that will decide its relationship with the European Union. If it does not, then prepare for more chaos ahead as Britain tests its democratic principles for the first time in more than a century.

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