AFTER MONTHS of mostly fruitless debate and maneuvering, Britain’s Parliament has finally embraced the best way out of the country’s impasse over leaving the European Union: a general election. It has been clear for some time that no majority exists in the current body for any one formula for carrying out the split that Britons narrowly voted for 3 1 / 2 years ago. Those voters ought to have another chance to weigh in, especially now that it is clearer what the consequences and costs of Brexit will be.

Unfortunately, the choice on Dec. 12 will be neither clean nor, to centrists, appealing. Prime Minister Boris Johnson and his Conservative Party will promise to quickly complete the break with Europe if they are given a majority. To his credit, Mr. Johnson earlier this month negotiated a new exit deal with Brussels that is more palatable than the “no-deal” crashout Britain appeared to be headed for when Mr. Johnson took office as prime minister in July.

Still, economists say the plan would leave Britain poorer over time by constraining trade with its largest partner. It would set up a potentially troublesome customs border between Northern Ireland and the rest of Britain. Northern Ireland, Scotland and even Wales might seek independence in an effort to remain in the E.U. Many who voted for Brexit in 2016 did not anticipate those outcomes; they were promised — by Mr. Johnson, among others — that Brexit would only shower the country with benefits.

The small Liberal Democrat party and the Scottish National Party oppose Brexit. But Labour, the main alternative to the Conservatives, has taken a muddled position, promising to negotiate a new deal with Brussels and then call a referendum on that deal. It leaves pro-Europe “remainers” with a difficult decision, particularly as Labour’s leader, Jeremy Corbyn, is offering a far-left agenda of nationalization and other socialist measures that would probably do more damage to Britain’s economy even than Brexit.

A Corbyn government could also poison transatlantic relations, even if President Trump leaves office. The Labour leader has a long history of antipathy toward U.S. foreign policy and flirtation with its enemies, including Venezuela’s Chavista regime and the Mideast militant groups Hamas and Hezbollah. Mr. Johnson, who has friendly relations with Mr. Trump, has a Churchillian view of the U.S.-British partnership and favors a free-trade agreement with Washington.

Those who wish for neither the economic and strategic diminishment that Brexit would impose on Britain, nor Mr. Corbyn’s radical agenda, can hope for a surge by the Liberal Democrats and other small parties. If Labour defeats the Conservatives next month but falls short of a majority, it might be forced into a coalition that would temper its agenda while allowing a new referendum on Brexit. However, Britian’s election system, which awards seats in individual districts to the first finisher in multicandidate races, tilts against the small parties.

All this means an election may not leave Britain better off, or even resolve whether and when it leaves Europe. It is, nonetheless, its best chance for ending its prolonged political crisis.

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