The FBI director has been seen as giving it credence. So has the head of the U.S. Drug Enforcement Agency. And Gov. Christie made his views clear last week, saying in a Fox News interview that "I absolutely believe it's real."

The subject of debate: the "Ferguson effect," a theory that officers are responding to backlash by pulling back from proactive policing, potentially resulting in increased crime.

Christie, making a pro-law enforcement pitch in his Republican presidential campaign, has endorsed the theory, which detractors - including President Obama - say isn't grounded in evidence.

Obama's "rhetoric has not been supportive at all of the men and women in uniform around this country," Christie said Monday at Camden County Police Headquarters. "And it's his own FBI director who has said this type of conduct has made a chill wind go through law enforcement across this country."

At a speech last month in Chicago, FBI Director James Comey said he had a "strong sense" that "some part of the explanation" for increased homicides in certain cities was a change in behavior by police, feeling under siege in "today's YouTube world."

In New Jersey, the head of the main police union said officers were wary of their actions being inaccurately portrayed. Individual departments, meanwhile, said nothing about their policing had changed.

The phrase "Ferguson effect" was used by St. Louis police to explain increased crime as police were diverted to control protests after a white officer shot and killed an unarmed black teenager in August 2014 in Ferguson, Mo. The officer was cleared earlier this year.

The term gained more traction after a May 2015 Wall Street Journal commentary by a fellow at the conservative Manhattan Institute. Titled "The New Nationwide Crime Wave," the piece noted rises in violent crime in a number of U.S. cities following protests over "highly publicized deaths" of unarmed black men.

"The most plausible explanation of the current surge in lawlessness is the intense agitation against American police departments over the past nine months," wrote Heather Mac Donald, contending the "onslaught of antipolice rhetoric" had spurred officers to scale back on proactive policing.

In his remarks, Comey said better data were needed to get a "full picture of what's happening."

Noting that some cities had no crime increase this year, Jeffrey Fagan, a professor at Columbia Law School, said there was "no evidence other than a few unconfirmed anecdotes that there is de-policing in U.S. cities, or that police actions have led to a rise in homicide in a handful of cities."

An analysis in September by FiveThirtyEight found that in 26 of the 60 largest U.S. cities, homicides were up 20 percent or more from a year ago; overall, homicides were up 16 percent. In 20 of the 60 cities, homicides were down. The website used data from police departments and news reports, depending on the information available.

FiveThirtyEight said the homicide increase "doesn't come close to reversing the long-term decline in homicides."

While Christie has said the Ferguson effect is real, he has also said it isn't happening in New Jersey.

"I don't see it in New Jersey because the leader of New Jersey tells the police officers go out and do their job, without exception," Christie said last month on CBS's Face the Nation.

Several New Jersey police departments also denied any impact. "The so-called Ferguson effect has had no impact here in Newark because we have been fortunate to never have had the issues plaguing the Ferguson Police Department as far as race relations are concerned," Police Director Eugene Venable said. Police "have taken an oath to protect and serve, and that's exactly what the officers in our department do, day in and day out."

The U.S. Department of Justice last year reached an agreement with Newark to have a federal monitor watch over the police, after a review found the Newark department had repeatedly violated residents' civil rights.

In Camden - where the city department was disbanded in 2013 and replaced by a county force that Christie has credited with reducing crime - county spokesman Dan Keashen said officers had a "baseline priority" of "community interaction and human contact with the public."

"We have not seen a chilling effect of any kind, and believe that body cameras and accountability are the key to maintaining good will throughout the city," Keashen said.

Some said officers couldn't help but be affected by the national focus on policing. "The whole Ferguson issue - anybody who says it hasn't changed our jobs has their head in the sand," said Patrick Colligan, president of the New Jersey State Policemen's Benevolent Association. "Everybody seems to watch a police episode of Cops, and they're all an expert on how we do our jobs."

Officers are afraid of "national pressure" from an out-of-context video that's gone viral, Colligan said - citing the sheriff's deputy fired in Columbia, S.C., last month after he flipped a girl out of a desk at school and dragged her away.

"We don't see the whole video," Colligan said. "I'm not saying every officer would have handled it that way, but what do you do?" The federal justice department has opened a civil-rights investigation into the incident, which involved a white officer and black student.

If police are following policy and "treating every citizen fairly . . . I don't really see how it would be an issue," said Richard Smith, president of the New Jersey NAACP, which has been working with police departments to host community forums and last month hosted training on implicit bias for law enforcement officials.

In July, Christie's administration announced plans to equip all state police patrol officers with body cameras, dedicating $1.5 million to buy 1,000 cameras. Another $2.5 million was announced for local departments looking to buy cameras.

The state police have had in-car cameras for years, as have some local police departments.

"My officers know, even prior to Ferguson, everything they do is recorded," said Palmyra Police Chief Scott Pearlman, president of the Burlington County Police Chiefs' Association. While "I'm sure they're concerned whatever incidents they get involved in could be the next YouTube situation," he said, "I don't see my officers slowing down" in their policing.

mhanna@phillynews.com

856-779-3232

@maddiehanna

www.philly.com/christiechronicles