When Trump closed the gap in the waning days of the 2016 campaign, he had to persuade very few Clinton supporters to vote for him. Trump just had to get support from those who were undecided or supporting a third-party candidate.
At the moment, Trump’s path will be difficult if not next to the impossible to win if he does not get back the voters who are already with Biden.
Other historical examples are not very kind to Trump either. The only other challenger at this point who was or was hovering around 50% was Jimmy Carter in 1976. He won.
Here’s the problem for Trump: The average poll at this point had Truman’s Republican opponent Thomas Davie at 46%. So even when Biden’s lead over Trump in those polls is less than 2 points higher than Dewey was above Truman (just south of 9 points), Biden gained about 5 points more support than voters. Dewey simply did not have the voters to commit to him in the same way that Biden does at this point.
Indeed, Trump’s need to persuade those who are no longer with the other camp is reflected in a different way. His rating in the middle poll is 54%. No other president at this point in the election era had such a high acceptance rating before the opportunity to win a second term. Truman’s was 47%, with his approval rating dropping to 39%. There was, however, the majority of Americans who at least did not disapprove of him.
Trump proved in 2016 that he could defy history. If he is to win a second term in 2020, Trump will have to do some.