What was the great upset in 1948 election?

With regards to US history in presidential elections, many people believe that the election of Harry S. Truman in 1948 was a remarkable upset. Even newspapers had already printed that Truman had lost to the Republican candidate, the governor of New York, Thomas Dewey. Dewey was heavily favored and being from New York was a big plus for him. Nevertheless Dewey lost, giving Truman a huge upset victory.Moving on to the 21st century, the upset victory of Donald Trump over Hilary Clinton was a huge political upset.