here is the entire allegory.
- The Wizard of Oz is William McKinley who represents all politicians - full of hot air and not much action
- the Scarecrow who has no heart because he represented all farmers who were not rotating their crops and therefor depleting its nutrients - so hence no brain
- the Tin Man as he was emblematic of all factory owners who pushed children to work and had people at it six days a week and so they had no hearts and so it goes.
- Just read the link above to get the full story.