The sign was hanging in McCain's Pompano Beach, Fla., campaign office Thursday.
After being shown a picture of the sign, Broward County Republican Chairman Chip LaMarca said he was "disgusted" and would immediately remove the sign, the newspaper reported.
"I'm speechless at the ignorance," LaMarca said. "It's not something we can condone. I'll find out who put it up there, and maybe they'll volunteer somewhere else."
Jeffrey Sanders, 23, told the newspaper he saw the sign Thursday when he stopped by the campaign office to pick up tickets for a McCain rally at Florida International University.
"To label someone as the next Hitler is beyond outrageous (and) offensive. And it needs to be addressed. And if Barack Obama does the same thing to McCain, I would expect the same reaction," he was quoted by the newspaper as saying.