Speaker Nancy Pelosi has begun a formal impeachment investigation of President Trump, saying he “must be held accountable.” We spoke to our colleague who was at the announcement and to one of the lawmakers who helped convince Ms. Pelosi that it was time. Guests: Nicholas Fandos, who covers Congress for The New York Times, and Representative Mikie Sherrill, Democrat of New Jersey. For more information on today’s episode, visit nytimes.com/thedaily.  Background reading: Though the outcome is uncertain, the inquiry raises the possibility that Mr. Trump could become only the fourth president in American history to face impeachment.After months of caution from House Democrats, why is this happening now? “They believe the new accusations against Mr. Trump are simple and serious enough to be grasped,” our colleague Carl Hulse writes in a news analysis.Here’s how the impeachment process works.