Glenn Rushing, chief of staff to Rep. Sheila Jackson Lee, D-Texas, told the Houston Chronicle the cartoon -- depicting two L-shaped gallows with "Sheila Jackson Lee" written on one and "Hussein Obama" on the other -- arrived at her Houston office Saturday.
On the House floor Friday, Lee accused Republicans of "maligning and maliciousness" toward Obama in debt negotiations.
"Why is this president being treated so disrespectfully?" she asked. "I am particularly sensitive to the fact that only this president -- only this one, only this one -- has received the kind of attacks and disagreement and inability to work, only this one. Read between the lines."
Rushing referred the fax to the House sergeant-at-arms and Secret Service and FBI directors.
Secret Service spokesman Robert Novy said: "We are aware of the cartoon, and we are taking appropriate measures. Being a protective intelligence matter, the Secret Service has no further comment."