Wisconsin, Pennsylvania, Arizona, Florida: The 9 counties that could decide a presidential election

Lawn signs urging people to vote for Democratic presidential nominee Joe Biden, left, and President Donald Trump in Newburg, Wisc., on Sept. 18, 2020. Wisconsin is one of the “blue wall” states that Trump won in 2016 — to the surprise, chagrin and embarrassment of the Clinton campaign. (Lauren Justice/The New York Times)