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. He ascended to the office through the vice-presidency, after Franklin Roosevelt died in 1945.
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.
George H.W. Bush in 1992 was the other elected incumbent to lose reelection since 1940. At this point, he was ahead of Bill Clinton, though he found himself trailing in a number of polls to independent Ross Perot.
Both Bush and Carter had similar approval ratings to Trump now.
Elected incumbent races are much more likely to be referendums on the occupant of the White House. Those without them tend to be much more of a choice between the two major party candidates.
Indeed, the other presidential candidates to blow clear leads at this point were running in non-incumbent races.
In many of these non-incumbent races, a lot of voters have weak or no opinions about one or both of the candidates.
Opinions of Bush also were weak at this point in 1988. A sizable minority (41%) of voters told CBS News that they hadn’t heard enough or were undecided on Bush. In that recent NBC News/Wall Street Journal poll, just 9% said they were neutral or not sure about Trump.
In other words, four to five times fewer voters are undecided on the most important factor for the 2020 race (Trump) than they were for the two candidates in 1988.
The other big non-incumbent race where the clear polling leader at this point lost the popular vote was 2000. George W. Bush was up by 8 points and lost the popular vote to Al Gore. But about 3 times as many voters (25-30%) were undecided or had no opinion on Bush and Gore than do on Trump currently.
Right now, more voters have a strong opinion of Trump than any other candidate at this point in 40 years.
Simply put, Trump doesn’t have a lot of room to mount the type of comeback he needs to.