Pennsylvania holds the largest number of electoral votes among seven highly competitive US swing states. Harris and her Republican opponent, former President Donald Trump, have spent most of their time campaigning there.
"This could be one of the closest races in history. Every single vote matters," Harris said in Philadelphia on Monday night.
An AtlasIntel poll found on Sunday that Trump was ahead of Harris at least 1 percentage point in all swing states, with the largest gaps of 6.8 points in Arizona and 5.5 points in Nevada, and the lowest gaps between 1.3 and 1.7 points in Wisconsin, Michigan and Pennsylvania. Harris was also 1.8 percentage points behind Trump in Georgia and by 3.6 percentage points in North Carolina.