Treasury Secretary Janet Yellen spends a lot of her energy thinking about what might threaten the U.S. financial system. She wants to ensure that the market where the U.S. government borrows money is running smoothly. She’s worried about hedge funds that load up on debt. She’d like banks to be better prepared for a crisis. These days, there’s also another, more foundational item on her list: whether we’ll have a peaceful transition of power after the 2024 election. In a phone interview...