When Trump closed the gap in the final days of the 2016 campaign, he had to persuade very few Clinton supporters to vote for him. Trump only had to get support from those who were reluctant or support a third-party candidate.
For the time being, Trump’s path will be difficult if not next to impossible to win if he does not recover the voters who are already with Biden.
Other historical examples are not very nice to Trump either. The only other competitor at this point that was at or around 50% was Jimmy Carter in 1976. He won.
Here’s Trump’s problem: The average poll at this point was his Republican opponent, Truman Thomas Dewey, at 46%. So even as Biden outperformed Trump in these polls less than two points behind Dewey’s superiority over Truman (just nine points south), Biden gained about 5 points more than voters. Dewey simply did not have voters sticking to it in the same way Biden did at this point.
Indeed, Trump’s need to persuade those who are not already in the other camp is reflected in another way. His rejection rate in the poll is 54%. No other president at this stage of the polling period got a rating rejected this high before his chance to win a second term. Truman was 47%, with his approval level dropping at 39%. However, there was a majority of Americans who at least disagreed with it.
Trump proved in 2016 that he could challenge history. If he were to win a second term in 2020, Trump would have to win some.