donald trump 0625
CNN  — 

The polls are fairly unanimous: Former Vice President Joe Biden holds a significant advantage over President Donald Trump. On average, he’s up by 10 points nationally and in the pivotal battleground state of Wisconsin.

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.

The fact that the only incumbent to come back (Truman) is an indication of a wider phenomenon: races without an elected incumbent running for re-election are different from ones with one. Gerald Ford was the only president not given another term with a positive net approval (approve - disapprove) rating, and he wasn’t an elected incumbent.

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.

View Trump and Biden head-to-head polling

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.

Consider the 1988 campaign. Democrat Michael Dukakis was up by around 5 points in late June 1988. His lead over George H.W. Bush would grow even larger later in the summer. Yet, it was Bush who would win an 8-point victory in the fall.

Dukakis, though, was barely known at this point. About a majority (49%) of voters said they hadn’t heard enough or were undecided when asked whether they had a favorable or unfavorable view of him in a CBS News poll. Biden is much better known. In an NBC News/Wall Street Journal poll released earlier this month, 25% said they were neutral or not sure about Biden.

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.

View 2020 presidential election polling

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.

The fact that 1988 and 2000 moved a lot fit a larger pattern. When fewer voters have a strong opinion of the candidates, it’s correlated with more movement in the polls.

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.