Filner's lawyer Harvey Berger made the argument in a letter to City Attorney Jan Goldsmith on Monday, one day before the City Council unanimously voted to deny Filner funds for his legal defense.
Local media published parts of the letter Wednesday.
Seven of nine City Council members have joined former supporters in urging Filner to resign.
Filner's former communications director is suing the mayor and city, alleging the former congressman made unwanted sexual advances.
Seven other women have made similar allegations.