What is the origin of the journalistic term muckraker?

Shortly after the turn of the century, President Theodore Roosevelt said that the writers of exposes who flourished at the time reminded him of John Bunyan’s Man with the Muckrake.

The Man with the Muckrake when offered a heavenly crown, “would neither look up nor regard the crown he was offered, but continued to rake to himself the filth of the floor.”