Right now - Trump is ahead of Truman - Truman was down 13 by the 20th of August. Truman would narrow that to 6 by the last week of September.
HW Bush had recovered to 7 by August 15th.
Dubya was down by 10 by September 20th.
We'll know pretty quick where Trump stands. His campaign is in terrible historical shape. He has 2 weeks to shape up.
Big differences:
1. People did not understand how polls worked. Those were national polls, far more relevant than when America did not have the polarization that it had when Obama was running. We now pay attention to statewide polling.
2. Donald Trump is not Harry Truman. Truman had years of service in the military (he was a colonel) and in elective office. Truman was a problem solver as a politician, and he used that combination to get elected in 1948. He also faced an opponent who had serious weaknesses as a campaigner. Donald Trump insults huge swaths of the American public and gets into trouble for that.
3. Hillary Clinton now owns the successes of a President that the other Party has grossly underestimated. If anything, Hillary Clinton is riding the Obama legacy while having the formidable Obama campaign apparatus intact. She owns Obama successes and failures in foreign policy -- which is lots of successes and few failures. Obama foreign policy little differs from what Ronald Reagan got away with, which one can best describe as 'effective'.
Does this sound familiar? Obama as Reagan and Hillary Clinton as George Herbert Walker Bush is a fair analogy. Donald Trump is running on his questioned reputation of business acumen.
It's the Obama apparatus/Clinton campaign that will be running the equivalent of the "Willie Horton" ads this year.