The letter, a copy of which was obtained by The Inquirer, asks supporters to donate $100 or more to a legal-defense fund for Washington, who has denied any wrongdoing.
Felton, who is also a leader of the politically active Black Clergy of Philadelphia and Vicinity, called the charges "unfounded" and said Washington's "character is unblemished and her reputation untarnished, though she has practically been tried and convicted in the press."
Felton could not be reached Tuesday for comment.
Washington, 68, was arrested in March after a Montgomery County grand jury indicted her on charges of theft of services and conflict of interest. According to the grand jury presentment, Washington directed state Senate staff to plan her birthday party, which doubled as the primary fund-raiser for her political campaigns, every summer from 2005 to 2013.
The grand jury estimated that Washington used $30,000 to $100,000 worth of taxpayer money to plan the fund-raisers, which were usually outdoors, with catered food, live music, and themes such as "Celebration Under the Stars."
If convicted, she faces up to 12 years in jail.
The staff member who alerted authorities to the alleged misconduct also filed a lawsuit against the Senate Democratic leadership.
Sean McCray, the senator's former chief of staff, seeks more than $700,000 in damages, alleging wrongful termination, defamation, and emotional distress. McCray alleges the senator slashed his salary, berated him in front of office staff, and fired him for blowing the whistle on what he felt was an illegal use of staff time.
Washington lost the Democratic primary in May and will leave the General Assembly in December after 20 years representing portions of northwest Philadelphia and southeast Montgomery County.