LONDON — The Labour Party canvassers gathered after dark outside a tube station in Pimlico, a pocket of central London that, by all appearances, should be fertile terrain. Nearly three-quarters of the surrounding district voted to stay in the European Union, among the strongest “Remain” votes in Britain, putting the pro-Brexit Conservatives at risk in a seat they had held since the district lines were drawn in 1950.

But the district, the Cities of London and Westminster, with its rows of white stucco townhouses and crowded housing projects, may now become a parable on the left for why Prime Minister Boris Johnson holds a commanding position less than two weeks before the election.

Brexit has sent tremors through the British political system, shaking up the traditional left-right, class-based divisions. While the Conservatives have capitalized on the upheaval, building a coalition of pro-Brexit voters across regional and class lines, the left has so far struggled to win converts and overcome its own divisions.