Can Brexit Be Stopped?

BOURNEMOUTH, England—Standing onstage in front of her party faithful, Jo Swinson had a surprise. At the opening rally of the Liberal Democrats’ conference here at the British seaside, the crowd knew what to expect—all week, rumors had been buzzing that they would be joined by a high-profile defector from a rival party—but they didn’t know whom to expect. Even their own members of Parliament were kept in the dark until half an hour before the announcement.

Swinson, the party’s newly elected 39-year-old leader, said the person she was introducing was “someone who has stood up for liberal values,” and moments later, Sam Gyimah bounded up onstage. Gyimah, born in Britain and raised in Ghana, had once been hailed as the future of the Conservatives—the party of Boris Johnson, which is furiously trying to pull Britain out of the European Union. Just three months ago, in fact, Gyimah had stood for the leadership of the Conservative Party.

Now he was a Liberal Democrat.

In her introduction, Swinson stressed Gyimah’s social liberalism—the 43-year-old voted to legalize same-sex marriage in 2013—and his commitment to stopping Brexit. Gyimah ran for the Conservative leadership while calling for a second EU referendum, a position that proved about as popular with its grassroots members as a 100 percent inheritance tax or banning Downton Abbey. It was this experience, he said, that had persuaded him to leave the party. Now his move to the Liberal Democrats sent a clear message: The Conservatives had no place for pro-European social liberals anymore. To underline the point, Swinson was wearing a necklace bearing the word European, while Gyimah took the stage wearing orange trousers—one of the Lib Dems’ signature colors.

The past few months have been chaotic, even by the recent standards of British politics. After Theresa May failed to get her deal