Eight of the writers were sympathetic to Jackson, including one from the chair of the Congressional Black Caucus, Representative Marcia Fudge (D-Ohio).
She called Jackson the "highlight of our karaoke nights."
Another letter says: "Jesse Jackson, Jr. betrayed the public trust for his personal gain. His sentence should reflect that."
A suburban dentist wrote: "A strong message needs to be sent and an example needs to be made!"
Jackson pleaded guilty to misusing more than $750,000 in campaign cash.
He and his wife Sandi will be sentenced July 1.