Everything You Need to Know About the Order on Sanctuary Cities

President Donald Trump signed an executive order Wednesday singling out the country's sanctuary cities in an attempt to curtail what he has called "virtually unlimited immigration."

"Sanctuary jurisdictions across the United States willfully violate Federal law in an attempt to shield aliens from removal from the United States," reads the order. "These jurisdictions have caused immeasurable harm to the American people and to the very fabric of our Republic."

Critics of the president and advocates for undocumented immigrants were quick to criticize the order, with some calling it unconstitutional, while supporters lauded Trump for swift action on a campaign promise.

Here's what you need to know:

What Is a Sanctuary City?

There is no precise definition of a sanctuary city, but the term generally applies to any municipality willing to defy federal immigration laws in order to protect undocumented immigrants. The practice began in the 1980s with cities such as San Francisco seeking to ease the transition of immigrants and incorporate them into the community. Local laws and ordinances mandated that city services be provided to all citizens regardless of their immigration status and that no resources be solely dedicated to the discovery or apprehension of undocumented immigrants.

Since then, the term has evolved to include certain protections for undocumented immigrants. Some sanctuary cities refuse to allow their law enforcement officers to be deployed by U.S. Immigration and Customs Enforcement and others, like Los Angeles, prohibit police from apprehending or questioning persons for the sole purpose of learning their immigration status. Additional jurisdictions decline requests by ICE to detain undocumented immigrants pending transfer to ICE, according to the Immigrant Legal Resource Center.

Which Cities Are Sanctuary Cities?

A large number of major cities across the U.S. have designated themselves sanctuary cities, including Austin, Baltimore, Chicago, Dallas, Denver, Houston, Los Angeles, Miami, Phoenix, Portland, San Francisco and Seattle, among others. A number of counties and smaller cities and towns have also adopted sanctuary status.

Most notably, the two cities that President Trump calls home -- New York and Washington, D.C. -- are sanctuary cities. New York's status originated with an executive order signed in 1989 by then-Mayor Ed Koch. The order prevents local officials from "transmit[ting] information respecting any alien to federal immigration authorities" unless in connection with other criminal activity.

Washington, D.C., Mayor Muriel Bowser called a press conference Wednesday in response to Trump's executive order to reiterate the city's commitment to undocumented immigrants. She added that residents should not have to fear their deportation for actions such as calling 911 in an emergency or speaking to local police to report a crime.

What Does the Executive Order Do? Is It Legal?

Trump's executive order designates sanctuary cities as "ineligible to receive federal grants" should they continue to disobey laws regulating undocumented immigrants, a move that could convince these jurisdictions to give up their status. This means that cities could potentially lose out on millions of dollars in federal funds, though it is unclear which funds will be targeted. A November review of the New York City budget claimed the city receives $7 billion in total federal assistance across all programs.

Furthermore, the U.S. attorney general is directed to "take appropriate enforcement action," possibly in the form of legal proceedings, against sanctuary cities that do not comply, potentially stripping protections from undocumented immigrants.

The order, if carried out, will likely face legal hurdles. The Supreme Court previously set a series of rules in 1987's South Dakota v. Dole that mandate when federal funds can be curbed. One of the rules is that such funds can't amount to a sum that would be "coercive," therefore forcing the recipient to comply with the federal government. Should a court decide that the funds represent a total large enough that Trump's order is coercive, it could be ruled unconstitutional.

South Dakota v. Dole also dictates that the federal funds withheld must be related to the law. Therefore, Trump's order to suspend funding would likely be limited to federal grants related to immigration, like law enforcement, or else would be susceptible to legal action. It's further unclear whether the precedent set by South Dakota v. Dole regarding an act of Congress is applicable to executive orders.

How Are Cities Responding?

Following the signing of the order Wednesday, the leaders of several cities spoke out in opposition to the order.

"We're going to defend all of our people regardless of where they come from, regardless of their immigration status," said New York City Mayor Bill de Blasio. "The spirit of this executive order runs counter to our spirit and values as a city."

San Francisco Mayor Ed Lee added, "Nothing has changed. Our city is still a sanctuary city. We are going to remain a sanctuary city precisely because the purpose is to keep everybody safe."

Chicago Mayor Rahm Emanuel reaffirmed his city's status, saying, "I want to be clear, we're going to stay a sanctuary city. There is no stranger among us."

Leaders in Boston, Los Angeles and Seattle spoke out as well against the action Wednesday.