DAVID MUIR: Tonight, the high-stakes showdown here in Philadelphia between Vice President Kamala Harris and former president Donald Trump. Their first face-to-face meeting in this presidential election. Their first face-to-face meeting ever. LINSEY DAVIS: A historic race for president upended just weeks ago. President Biden withdrawing after his last debate. Donald Trump is now up against a new opponent. DAVID MUIR: The candidates separated by the smallest of margins. Essentially tied in the polls nationally. And in the key battlegrounds, including right here in Pennsylvania, all still very mu…