(CNN) Most times when we look at polling we talk about margins. For example, former Vice President Joe Biden leads President Donald Trump by a 7 point margin in an NBC News/Wall Street Journal poll out on Sunday. What that can miss, however, is that Biden's close to 50% (49%), while Trump isn't anywhere close (42%) in the poll.
Indeed, if you look at the average of live interview polls released since last Sunday, Biden manages to eclipse 50% (51%). That is, he has a majority for now. This is a key milestone that shouldn't be undersold.
As I've noted in the past, Trump's comeback in 2016 was made considerably easier by the fact that Democrat Hillary Clinton wasn't polling close to 50%. The average live interview poll taken in June 2016 (when Libertarian Gary Johnson was included) had Clinton at a mere 42%. Not a single one of those polls had her even touching 50%. In fact, she never got close in the average of polls during the rest of the campaign.
When Trump closed the gap in the waning days of the 2016 campaign, he had to convince very few Clinton supporters to vote for him. Trump merely had to pick up support from those who were undecided or backing a third party candidate.
Right now, Trump's road will be difficult if not next to impossible to win if he doesn't take back voters who are already with Biden.
Other historical examples aren't too kind to Trump either. The only other challenger at this point who was at or hovering around 50% was Jimmy Carter in 1976. He won.
One of the presidents who I often point to as a beacon of hope to Trump is Harry Truman. He is the one incumbent president who was trailing outside the margin of error at this point in his bid for a second term who came back to win. Moreover, Truman was the only president with a net negative approval rating (approval - disapproval) below -5 points at this point in the campaign to come back and win.
Here's the problem for Trump: The average poll at this point had Truman's Republican opponent Thomas Dewey at 46%. So even as Biden's advantage over Trump in these polls is less than 2 points greater than Dewey's was over Truman (just south of 9 points), Biden's earning about 5 points more support from voters. Dewey simply didn't have voters committing to him the same way Biden does at this point.
Indeed, Trump's need to convince those who aren't already with the other camp is reflected in another way. His disapproval rating in the average poll is 54%. No other president at this point in the polling era had a disapproval rating this high before their chance to win a second term. Truman's was 47%, as his approval rating languished at 39%. There was, however, a majority of Americans who at least didn't disapprove of him.
Trump proved in 2016 that he can defy history. If he is to win a second term in 2020, Trump's going to have to make some.