Britain’s prime minister should be triumphant when he addresses his Conservative Party’s conference in Manchester this afternoon. In May’s election he won an unexpected majority. The economy is steaming along nicely. And the opposition Labour Party has chosen a far-left leader, Jeremy Corbyn, whom even many in his own party consider unelectable. Yet Labour’s feebleness is also emboldening Mr Cameron’s Conservative critics—who are hard to ignore with a Commons majority of just 12. In particular, Eurosceptics accuse him of being ready to accept only minor changes to Britain’s European Union membership before recommending a positive vote in the forthcoming referendum. Europe has torn the party apart repeatedly before and could easily do so again. And Mr Cameron’s leadership is weakened by his pledge not to serve a third term as prime minister. Many in Manchester will be contemplating not what he says, but who might succeed him.