Promoting himself as a president who would crack down on "lawlessness," Gov. Christie has turned to a phrase that has been propelled to the forefront of the immigration debate.
"Sanctuary cities, engulfing Americans in crime," the Republican governor says over footage of police cars and flashing sirens in a campaign ad released last week.
As GOP poll-leader Donald Trump has accused Mexico of sending criminals and "rapists" across the border, his rivals have joined anti-illegal immigration activists in targeting so-called sanctuary cities.
The term has been used often following a fatal shooting of a woman this summer in San Francisco, allegedly by a man previously deported five times and whom local officials released from jail after federal authorities sought to have him detained.
While sanctuary cities have been a focus for a number of Republican presidential candidates - some of whom have called for cutting federal funding to the cities - there's no single definition of the term.
"One of the issues in having this national discussion about sanctuary cities is not everybody's talking about the same thing," said Kevin R. Johnson, dean of the University of California Davis School of Law.
Policies sometimes ascribed to "sanctuary cities" can range from barring police from questioning people solely to determine immigration status - as Los Angeles does - to not complying with U.S. Immigration and Customs Enforcement requests to keep people in custody beyond their release on local charges.
Under an April 2014 policy, Philadelphia does not comply with ICE detainer requests or notify ICE of pending releases unless a person is being released after a conviction for certain violent crimes and the detainer is supported by a judicial warrant.
The "sanctuary" label also isn't always used pejoratively: San Francisco, for instance, has described itself as having a "sanctuary" ordinance.
The varying interpretations associated with the term haven't deterred candidates from using it liberally.
Reacting to the San Francisco shooting, former Florida Gov. Jeb Bush decried "sanctuary cities that encourage this."
Louisiana Gov. Bobby Jindal called for "cracking down on the sanctuary cities," and proposed that mayors in such cities be held accountable as well as criminals: "Let's lock them up."
Christie, like some rivals, has called for federal money to be stripped from sanctuary cities. A bill that passed the U.S. House last month would cut off some funds to jurisdictions that don't fully cooperate with federal immigration officials.
Christie's call for defunding could pertain to cities and counties in New Jersey, depending on how "sanctuary cities" is defined.
At least two New Jersey cities - Newark and Princeton - and five counties have policies of declining at least some ICE detainer requests.
Newark's policy, adopted in 2013 and posted online by the Immigrant Legal Resource Center, says it aims "to enhance . . . partnership with its immigrant communities." It specifies there will be "no expenditure of any departmental resources or effort by on-duty personnel" to comply with an ICE request.
A number of the policies were adopted following a 2014 U.S. Court of Appeals for the Third Circuit ruling that said state or local law enforcement agencies weren't compelled to comply with ICE detainer requests. The case - which involved a U.S. citizen arrested on a drug charge, and then held on an ICE detainer in Lehigh County, Pa., after posting bail - was brought by the American Civil Liberties Union.
Following the ruling, the ACLU of New Jersey sent a letter to every county in the state, asking officials to revise their policies regarding ICE detainers - warning that jails across the country were facing lawsuits from people arguing that their detentions had violated their constitutional rights and that honoring detainer requests damaged relationships with immigrant communities.
Among counties that altered their policies following the ruling was Camden County, which will honor ICE detainer requests only if a warrant or court order also is submitted.
"An ICE detainer is viewed as a request unless approved or signed by a court," county spokesman Dan Keashen said. He said the county still informs ICE when "someone of interest" is being released from jail.
Other counties also are in contact with ICE upon releasing people from jail, said Ari Rosmarin, public policy director for the ACLU-NJ.
But a state directive could disqualify New Jersey from certain federal funding under the "Enforce the Law for Sanctuary Cities Act" that recently passed the U.S. House, Rosmarin said.
The directive, issued in 2007 by then-attorney general Anne Milgram, says that any local, county, or state law enforcement officer who arrests someone for an indictable crime or for driving while intoxicated shall ask about the person's citizenship, nationality, and immigration status. It prohibits law enforcement officers from asking the immigration status of a victim or witness - who "should not be discouraged from approaching police officers," the directive says.
The "Sanctuary Cities Act" would apply to jurisdictions that bar officials from "gathering information regarding the citizenship or immigration status, lawful or unlawful, of any individual."
Christie's campaign did not respond to questions on how he defines "sanctuary city" - or how such cities were increasing or promoting crime, as his latest ad suggests.
"Social science study after social science study" show that immigrants are "less prone to crime than U.S. citizens," said Johnson, of UC-Davis Law. "Sometimes the rhetoric used by some of the politicians doesn't really match the reality of what's happening."
Advocates for more restrictive immigration policies say cities that don't comply with ICE detainer requests are obstructing immigration enforcement.
"I can't think of a legitimate reason to prevent the deportation of noncitizens who have been involved in crime here," said Jessica Vaughan, of the Center for Immigration Studies in Washington. "This is a good way to take people out of the community who have been causing problems."
In the first eight months of 2014, more than 8,100 people sought by ICE in "sanctuary jurisdictions" were released, according to Vaughan.
During that time, about 1,900 re-offended, she said.
Vaughan defines a sanctuary jurisdiction as any that is "not complying with detainers, or in some way restricts communication between local law enforcement and immigration authorities."
Others apply a broader assessment. Steve Salvi, an Ohio man who created a "sanctuary city" list that appears to have been widely circulated online, includes a number of New Jersey cities without formal policies.
"I also look at behavior," Salvi said, asked to explain his criteria. He started his list in 2007 and said his website got "a quarter-million clicks" in July. "It spiked, definitely."