Complex sentence: The legend behind the second-term curse is that after Franklin D. Roosevelt broke the "de facto" second term limit by running for third and fourth terms, the ghost of George Washington might have put a curse on any president who seeks a second term.
Split into simpler sentences: The legend behind the second term curse is the fact that Franklin D. Roosevelt broke the "de facto" second term limit by running for third and fourth terms. Some people have said they believe that, in response, the ghost of George Washington might have put a curse on any president who seeks a second term.