“Police chiefs definitely have a shorter shelf life post-Ferguson, and they are more scrutinized and criticized for things they weren’t held responsible for before,” said Charles McClelland, Houston’s police chief, who said he agreed with the change in focus. “Pre-Ferguson, you were held responsible only for the crime rate — now it is community relations. One egregious act of misconduct by a police officer can get a police chief fired. It wasn’t like that before.”

As if in acknowledgment of the changed atmosphere, the authorities in San Bernardino, Calif., made officers quickly available to the news media after the department’s quick response to the terrorist shooting there last week, which killed 14 people.

The job of police chief has changed partly because of the dramatic reduction in violent crime since the 1990s, which, especially after the Ferguson shooting, has led the public to demand that departments shift their focus to police accountability.

“Since crime has been so low for so long, there are very high expectations in terms of what people expect of police chiefs,” said Inimai M. Chettiar, the director of the justice program at the Brennan Center for Justice, a nonpartisan public policy institute affiliated with the New York University School of Law. “Not only are they expected to keep down crime, but now they are expected to treat people with courtesy. That is new.”

Mr. Batts and Mr. McCarthy have been prominent public advocates for improving police departments by pressing for greater accountability. So has Philadelphia’s police commissioner, Charles H. Ramsey, who plans to retire next month. The loss of these three well-known spokesmen has led some to fear that nascent efforts elsewhere will stall.

“When you are losing people the caliber of chiefs like Ramsey and McCarthy, there will be a void in leadership, and so I think some of these reforms will slow down,” Chief McClelland said. “You have to be bold to be a reformer, comfortable and confident, and I think that some younger police chiefs are scared to take those risks.”