Former 538er Harry Enten, who now writes for CNN, reflects on Trump's miserable polling relative to other incumbents:
There is time for Trump to mount a comeback, but the candidates who do come back are usually not incumbents and have never been elected incumbents in the polling era.
Since 1940, the only incumbent losing at this point in the cycle who would go on to win another term was Harry Truman. He, like Trump, was down around 10 points to Thomas Dewey in the early summer of 1948. But remember, Truman was not elected president before taking the 1948 election.
In terms of elected incumbents, Jimmy Carter was the one to be down by as much as Trump is right now. Carter went on to get crushed by Ronald Reagan in 1980.
The interesting point about Truman's "surprise" victory in 1948 is that it should not have been a surprise.
It became one mostly because all but one major pollster had stopped polling toward the end of October, since a Dewey victory seemed certain. In addition, Dewey's October lead was not that impressive; it averaged only 3.5 points. Last, the one polling operation — at the University of Michigan — that continued surveying likely voters discovered that undecideds were breaking for Truman. (That was similar to the joker in the deck in 2016's polling. As the election neared, polls showed that roughly 5 percent of voters were undecided, most of whom wound up choosing Trump. The polls weren't wrong, as is commonly believed. Clinton's loss was more the matter that nearly all observers — including myself — concentrated their predictions on her consistent, 2-3 point leads.)
President Carter's 1980 loss to Reagan was a Greek tragedy. Unlike Trump's tenure, in which all its wounds have been self-inflicted, Carter was battered by uncontrollable outside forces, mostly the energy crisis and, of course, the seemingly endless hostage situation in revolutionary Iran. The Carter campaign had prayed that Reagan would be the Republican nominee, since his reputation as a far-right wingnut remained firm. But the hostage crisis unfairly left Carter looking weak, which sank any hope of a win.
As for 2020? One merely needs to rephrase Mr. Enten's words: "In terms of elected incumbents, Donald Trump is the one to be down by as much as Carter was in 1980. Trump will go on to get crushed by Joe Biden."