Former President Donald Trump and Vice President Kamala Harris will face off in Philadelphia on Tuesday night for the second presidential debate of 2024. It will be the first time the two candidates meet on a debate stage. They enter the debate in a neck-and-neck race, with Mr. Trump leading Ms. Harris, 48 percent to 47 percent, according to the latest national polling from The New York Times and Siena College. That means the people still on the fence — those unsure about whom to vote for or whether to vote at all — are potentially the most important audience for the debate. Today, “The Run-Up” talks with Ruth Igielnik, a Times polling editor, about the 5 percent of voters who are still undecided. We then speak with four undecided voters to ask what they are hoping to hear tonight.