In general election surveys, Biden is improving in states with non-college whites to a statistically-significant degree. According to exit polls, this segment of the electorate was integral to Trump's win. White Americans without college degrees have been trending towards the GOP for many years, voting for Trump in 2016 by a historically-large margin. A reversal of this trend bodes well for Biden, who is outperforming Clinton the most in states with a higher-than-average population of non-college whites.
Supporting this trend, a breakdown by region also demonstrates that Biden is polling better than Clinton within groups integral to Trump's electoral college majority. Biden has been polling 8.4 points better in the Midwest than Clinton ended up doing in 2016. In comparison, Biden's increase on Clinton's margin averages 5.4 points and a median of 5.0 points across all states.