Appearing on CNN's "State of the Union," Feinstein said Filner, a former member of California's Democratic congressional delegation, was in a position that required him to be a role model.
"And I don't think that somebody who is lacking a moral compass really sets a role model or really will provide the kind of leadership that San Diegans want," said Feinstein.
Filner -- accused by at least seven women of improper sexual conduct -- conceded Friday his behavior has been unacceptable but has resisted demands, including some from political allies, that he resign. Instead, he said he will take a two-week leave of absence beginning Aug. 5 for therapy and then undergo regular counseling afterward.
Feinstein told CNN it would be easier for Filner to depart voluntarily rather than subject the city to a recall election.
The veteran senator and former mayor of San Francisco questioned Filner's judgment, particularly given his long political career.
"Of all people, Bob Filner knows what public life is like," she said.
"Being the mayor of a big city, you're a role model for people," Feinstein added. "You are either inspirational to people or you aren't. It's a very tough job."