In the last week, I’ve been crisscrossing the United States, meeting with politicians and their advisers ahead of this year’s congressional elections. Almost everyone has asked me about the ongoing Boris Johnson saga. Their most common refrain is surprise: how can the simple act of attending a party prompt a prime minister’s ever-more-likely political demise? In a country where every scandal bounced off Donald Trump, it is hard for Americans to imagine a few sausage rolls and glasses of warm wine in Downing Street causing so many to turn on the Prime Minister.