Britain on the brink

Tomorrow’s British election — one of the most important in a generation — comes as the bitterly divided country sits on the precipice of a decisive break with the European Union.

National polls show the governing Conservative Party with a tight and narrowing lead, and the opposition Labour Party gaining more support. Yet predictions are perilous: Narrow swings in voter preferences can produce major shifts in places where victory margins will be small.

As the campaign hurdles to a finish, we’re chasing these threads:

Health care: Strains on Britain’s beloved National Health Service — and the question of whether Brexit would exacerbate them — have jolted the national conversation at the last minute. The N.H.S. has deteriorated in recent years under the Conservatives’ watch, and Labour argues that Prime Minister Boris Johnson’s Brexit plan would undermine the system further.

Disinformation: The campaign is awash in doctored videos, dodgy websites and manipulated Twitter accounts. Some of the material comes not only from shadowy groups or Russian operatives, but also the political parties and candidates themselves, particularly the Conservatives.

Sick boy scandal: During a television interview on Monday, Mr. Johnson at first refused to look at a picture of a 4-year-old boy lying on the floor of an overcrowded Yorkshire hospital. The picture and the boy’s story — first reported by a local newspaper — were real. But Mr. Johnson’s interview spawned a social media campaign to discredit the boy’s family.