In 1980, Jimmy Carter, after a successful convention, was tied with Ronald Reagan according to the national polling average.2 But Reagan ended up winning by nearly 10 percentage points. Reagan’s gains were greater than the deficit Trump faces now.
Just four years before Reagan’s victory, Gerald Ford nearly pulled off the greatest comeback in the modern polling era. Ford was down by double digits in 1976. But aided by a decent job approval rating and perhaps by Carter’s missteps, Ford narrowed Carter’s lead and even inched ahead in the final Gallup poll of the campaign. Ford didn’t win another term, but he proved that a race can change even after both conventions are in the rearview mirror.
Perhaps the most interesting potential precedent for the 2016 campaign is 1968.3 Richard Nixon was up by 8 percentage points and opened an even larger lead in the fall. But Hubert Humphrey was able to consolidate a previously divided Democratic base (as Trump needs to do with Republicans) and cut into Nixon’s lead. Humphrey was also aided by President Lyndon Johnson’s rising approval ratings and the original October surprise (an announcement by Johnson that the U.S. was halting the bombing in Vietnam). Humphrey ended up losing in the second-closest presidential election of the 1900s. It’s not too difficult to imagine Republicans rallying behind Trump — perhaps Clinton is hit with a scandal or WikiLeaks drops an October surprise.