5 storylines heading into tonight's presidential debate

Who prepared better? Clinton's longtime aide Philippe Reines said the former secretary of state -- who is known for doing her homework -— has been poring over briefing books and practicing in mock debates with a Trump stand-in. Trump was in debate prep meetings all day Friday after focusing more on campaigning than studying up for the showdown. Tonight will be the first time that Donald Trump has ever debated an opponent one-on-one. Trump performed well during the GOP debate season with as many as eight to 10 other Republicans on the stage, but tonight will be different. As Politico's Shane Goldmacher observes, history has shown that most memorable moments of a debate typically happen early. Viewers should be glued to their TV screens for the first 30-40 minutes of the debate. Lester Holt from NBC's Nightly News will moderate tonight's debate and he's under intense pressure after colleague Matt Lauer was widely criticized following a Commander in Chief forum. NBC's Dante Chinni reports, "Looking at pre-debate NBC News/Wall Street Journal presidential polls and the final election results since 1992, there is only one campaign where the debate may have made a serious difference — 2000. In every other case, the candidate that led going into the debates wound up winning on Election Day."