https://fivethirtyeight.com/features/why-bidens-polling-lead-is-different-from-clintons-in-2016/
First, Biden’s lead has clearly widened in the past month. He now leads by more than 9 points, but on May 25, Biden led by an average of only 5.8 points (48.9 percent to 43.1 percent).
But some people have dismissed Biden’s lead by pointing out that Hillary Clinton also led in most polls of the 2016 election (Clinton, obviously, ended up losing to Trump). While this is true, Clinton’s lead was much smaller. Applying our current polling-average methodology to 2016 polls, Clinton led national polls by an average of about 4.0 points four months before the 2016 election, and 3.8 points on Election Day itself. So while a normal-sized polling error was enough to throw the 2016 election to Trump, it would take a much bigger — and much unlikelier — polling error for Trump to be ahead right now.