Sanctuary cities have long been in President Donald Trump's crosshairs. The term refers to the scores of cities and counties across the United States that limit their cooperation with federal immigration authorities by refusing most requests to detain, pursue or report undocumented immigrants (those here illegally) who have had contact with local law enforcement.

Just days after taking office, Trump signed a sweeping executive order making good on his key campaign pledge to crack down on illegal immigration. In addition to authorizing the construction of a border wall with Mexico and beefing up immigration enforcement, the order also threatens to cut billions of dollars in federal funds from so-called sanctuary cities.

"Sanctuary jurisdictions across the United States willfully violate Federal law in an attempt to shield aliens from removal from the United States," the order states. It warns that those jurisdictions that don't comply with federal immigration enforcement efforts will lose federal funding. "These jurisdictions have caused immeasurable harm to the American people and to the very fabric of our Republic."

Trump's order instructs federal immigration authorities to target a broader group of immigrants for deportation. It calls for the removal of immigrants "who have committed acts that constitute a chargeable criminal offense" or pose a public safety risk.

In response to Trump's 2016 election victory, mayors and police chiefs in more than 10 major cities, including San Francisco, Oakland, Los Angeles, New York, Chicago and Washington, D.C., reaffirmed their commitment to upholding sanctuary policies, and have continued to stand by those positions in the face of the recent order.

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"We'll proudly stand as a sanctuary city -- protecting our residents from what we deem unjust federal immigration laws -- fight all forms of bigotry and advance our commitment to equity even more passionately," Oakland Mayor Libby Schaaf stated shortly after the election.

San Francisco Mayor Ed Lee reiterated that sentiment on Jan. 26 in his State of the City address: “We are a sanctuary city now, tomorrow, forever.”

On Jan. 31, San Francisco became the first city to sue Trump over his order to defund sanctuary jurisdictions. The lawsuit argues that the order is an unconstitutional overreach of the president's power, in violation of the 10th Amendment, which it says protects the sovereignty of local jurisdictions.

“President Trump’s executive order tries to turn city and state employees into federal immigration officers. That is unconstitutional,” San Francisco City Attorney Dennis Herrera said when announcing the suit. “No president can commandeer the local police force and turn it into the deportation arm of the federal government.”

San Francisco receives nearly $500 million a year in federal funds, which would be at risk if Trump's order survives in court.

Although U.S. Immigration and Customs Enforcement (ICE) relies on local authorities to help enforce immigration laws, a 2013 federal appeals court decision concluded that those local agencies are not legally required to detain undocumented immigrants when requested to do so.

An estimated 11 million undocumented immigrants currently reside in the United States.

What are sanctuary cities?

There's no official legal definition, and what it means varies from place to place. Generally speaking, local law enforcement in sanctuary cities or counties don't ask or report the immigration status of people they come into contact with.

A sanctuary jurisdiction typically refuses requests from federal immigration authorities to detain undocumented immigrants apprehended for low-level offenses. For example, when someone gets arrested for a DUI, he or she might spend the night in jail, get processed and then released. If this person is undocumented, though, federal immigration authorities would be alerted and may ask local officials to hold this person for longer, and possibly deport them. A city or county with a sanctuary policy would generally deny that request unless legally mandated to do so.

How many sanctuary cities are there?

Depends who you ask.

A 2006 Congressional Research Service report listed 32 counties and cities with explicit sanctuary ordinances. A number of cities have adopted similar resolutions since then, including Berkeley, Oakland and East Palo Alto.

In an analysis of data from the Immigrant Legal Resource Center, the New York Times tallied 39 cities and 364 counties across the country that in some way limit how much local law enforcement can cooperate with federal detention requests. It's unclear, however, how much action some of these jurisdictions have taken, other than officially expressing opposition to what they consider harsh federal or state immigration laws.

And organizations in some municipalities even challenge the label. In 2011, for instance, the Los Angeles Times editorial board denied that Los Angeles was a sanctuary city, even though in 1979 the city had enacted a measure to keep local police from inquiring about the immigration status of those arrested, one of the first cities in the country to do so.

Note: not all metro areas shown here are necessarily sanctuary cities.

Four states -- California, Rhode Island, Vermont and Connecticut -- have also enacted ordinances in recent years that limit compliance with federal immigration officials.

A least nine Oregon counties in 2014 stopped complying with ICE requests to hold undocumented immigrants in jail for the sole purpose of deportation. The change came after a federal judge ruled that one of those counties violated a woman’s Fourth Amendment rights by detaining her without probable cause. Some legal experts say the ruling may spur more local sanctuary policies across the state and possibly nationwide.

Are sanctuary cities more dangerous?

On the campaign trail, and now as president, Trump has consistently claimed that sanctuary cities "breed crime" and have resulted in "so many needless deaths," with the underlying suggestion that undocumented immigrants are more prone to violent criminal activity.

The language in his recent executive order underscores this:

"Many aliens who illegally enter the United States and those who overstay or otherwise violate the terms of their visas present a significant threat to national security and public safety."

Crime statistics, however, suggest otherwise.

A recent analysis of FBI crime data by UC San Diego political science professor Tom Wong found that most counties considered “sanctuary” jurisdictions have notably lower rates of all types of crime, including homicide, than comparable non-sanctuary counties. In 2015, large metro sanctuary counties had 654 crimes fewer crimes per 100,000 people than large central metro non-sanctuary counties, the report found. That's nearly 15 percent less crime in sanctuary counties.

In smaller counties, the discrepancy was similar. The main exception, the report found, was in medium metro areas and counties bordering on large metro areas, where crime rates in sanctuary jurisdictions were slightly higher.

The report, it should be noted, was published by the Center for American Progress, a progressive policy organization. And it shows correlation, not causation. In other words, there no definitive proof that sanctuary policies actually cause lower crime rates (it could just be a coincidence).

Another study by the American Immigration Council, a pro-immigrant group, analyzed FBI and census data from 1980 through 2010. It found that among men ages 18 to 49, immigrants (both legal and illegal) were far less likely than native-born Americans to engage in criminal behavior or to be incarcerated.

Now keep in mind that both of these studies were conducted by left-leaning organizations with progressive agendas. And even though the conclusions are based squarely on federal statistics, skeptics are likely to counter that the authors used selective data to produce desired results.

Nevertheless, several national policing associations seem to have embraced these findings. As the Washington Post recently reported, the Major Cities Chiefs Association, representing the 63 largest urban areas in the nation, stated in a 2006 report, that “immigration enforcement by local police would likely negatively affect and undermine the level of trust and cooperation between local police and immigrant communities,” which would “would result in increased crime against immigrants and in the broader community, create a class of silent victims and eliminate the potential for assistance from immigrants in solving crimes or preventing future terroristic acts.”

The International Association of Chiefs of Police reiterated this position, writing that “state and local law enforcement should not be involved in the enforcement of civil immigration laws since such involvement would likely have a chilling effect on both legal and illegal aliens reporting criminal activity or assisting police in criminal investigations.”

Which California cities have "sanctuary" policies?

Note: Although some of these cities may not explicitly identify as "sanctuary cities," they've all adopted some type of policy (an ordinance, resolution or law enforcement directive) that limits how much local law enforcement officials can cooperate with federal immigration enforcement efforts. This is not necessarily a complete list - it only includes those cities for which official documentation could be found.

What's the history?

The roots of the modern sanctuary movement date back to the 1980s. U.S. churches, synagogues and other religious institutions began to provide refuge and services to thousands of undocumented immigrants from Guatemala and El Salvador who had fled civil unrest at home but were denied sanctuary in the U.S., largely due to Cold War politics.

The effort became known as the Sanctuary Movement, and as it spread, a number of cities throughout the country joined in solidarity, passing resolutions to overlook the immigration status of residents.

What are the arguments for and against these policies?

Supporters argue that cities have bigger public safety priorities and too few resources to handle immigration enforcement. Additionally, many local policymakers and law enforcement agencies argue that immigration enforcement is not their responsibility, and that cracking down on undocumented residents would undermine community relations, disrupt services and dissuade those residents from cooperating with crime prevention effort. They also note that none of their protective policies in any way prevent local police from pursuing immigrants suspected of committing crimes.

Trump is among a large number of mostly Republicans opposed to sanctuary policies, arguing that they encourage illegal immigration, undermine federal enforcement efforts and severely compromise public safety, resulting in crimes that could have been avoided through deportation.

What's unique about San Francisco's law?

Although the majority of sanctuary cities don't ask residents about their immigration status and refuse to share information with Immigration and Customs Enforcement (ICE), San Francisco is among a handful of localities that take things a bit further.

The City and County of Refuge ordinance, adopted in 1989, prohibits the city from using any "funds or resources to assist in the enforcement of federal immigration law or to gather or disseminate information regarding the immigration status" of residents unless explicitly required by federal or state law or court order.

The motion was further emphasized by a 2007 executive directive prohibiting city employees or agencies from assisting in any ICE investigation, detention or arrest proceeding unless required by federal law. And a section in the city's administrative code prevents any city law enforcement officer from detaining an individual "on the basis of a civil immigration detainer after that individual becomes eligible for release from custody.”

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Similar to other sanctuary cities, exceptions apply to individuals convicted of violent felonies within the past seven years or in custody for another violent felony.

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'Sanctuary Cities' Explained