Donald Trump starts 2020 in the worst polling position since Harry Truman (CNN link): A NBC News/Wall Street Journal poll finds that former Vice President Joe Biden leads President Donald Trump 52% to 43% in a general election matchup. An average of all polls this month puts Biden's advantage at a similar 7 points.
And the President starts out in a very unusual place for an incumbent: behind. Trump is the first incumbent president to be trailing at this point in the general election cycle (i.e. late March in the election year) since Harry Truman in 1948.
Polling at this point in the general election cycle when an incumbent is running is correlated with the ultimate outcome. A candidate in Biden's position would win the popular vote about two-thirds of the time if historical trends hold.
Moreover, there's something to be said about the consistency of Biden's edge. Despite the ever-shifting news cycle, Biden's lead in the average of polls has been between 5 and 10 points throughout the last year. In other words, Trump's general election polling has stayed stable, just like his approval ratings.
This fits a pattern of general election polling being less volatile than it used to be. You saw it in 2018, when Democrats held a consistent edge on the generic congressional ballot, which translated to them taking back the House. The polling at this point in the last two presidential elections in which an incumbent was running matched the final result within 0.3 percentage points.
Still, Biden, at this time, clearly has the advantage in the electoral college. Biden holds leads of 4 points or more in Arizona, Michigan and Pennsylvania. Add those states together with the states Hillary Clinton won in 2016, and Biden gets more than 270 electoral votes.
This reaffirms something that the 2018 midterms showed: it's very difficult to overcome a 7-point deficit nationally even if you're doing better in the electoral college. If all the states voted the way they did in the 2018 midterms for the House, Biden would easily defeat Trump in the electoral college.