… The 2020 race has been unusually stable, with Biden holding a healthy polling lead for months. … In the 2016 presidential race, Post-ABC polling was more variable. From June to October, there was a 9-point spread in the margin between Trump and Hillary Clinton in our polls, from a six-point to a 15-point Clinton lead. This year, the spread has been five points, from a 10-point to a 15-point Biden lead.
That persistent lead is a function of a number of important shifts in support since 2016. We took demographic data from each poll and averaged the margins among each group for the periods after the party nominees were set. Here’s how Trump’s position has changed with each. CONT.
Philip Bump, Washington Post