Another candidate, former judge Pat Lykos, hours later joined the call for Rosenthal to resign and criticized Siegler's remarks.
"It is interesting that Kelly Siegler has been part of the district attorney's office working side by side with Chuck Rosenthal for years, and now when Mr. Rosenthal is a political liability, she tries to separate herself from him and demand his resignation," Lykos said. "It is political theatrics and skilled tactics."
Monday was the first time that Siegler, who volunteered in Rosenthal's 2000 and 2004 campaigns and rose to chief of his special crimes bureau, took the stance that Rosenthal should quit now. His four-year term runs through this year.
"I can change what needs to be changed in that office and ... can start tomorrow because I know everybody there," Siegler, an assistant district attorney for 21 years, told the Memorial West Republican Women.
Defense lawyer and former prosecutor Jim Leitner, another candidate for district attorney, already has said Rosenthal should step down. Candidate and police Capt. Doug Perry said Rosenthal should follow his conscience.
The Republican primary winner will face Democrat C.O. Bradford, a former Houston police chief, in the November general election. Bradford, who was prosecuted by Rosenthal's staff on perjury charges that a judge later dismissed, declined to comment Monday.