President Trump signed two executive orders about immigration today at the Department of Homeland Security: one, putting plans in motion to build a border wall between the United States and Mexico, and two, taking action against sanctuary cities—municipalities that have refused to crack down on illegal immigration—that would strip them of federal funding.

It’s part of a wider rollout of immigration policy this week expected to address border security and refugee programs.

With many of the country’s major urban centers adopting sanctuary city policies, and the new administration threatening to withhold federal funding for those that don’t assist in federal immigration enforcement, the issue has repercussions beyond the immigration status of the roughly 11 million undocumented people in the United States.

.@PressSec says Trump administration will "strip federal grant money from sanctuary states and cities" that harbor undocumented immigrants. pic.twitter.com/vAOhL9V7cs — CBS News (@CBSNews) January 25, 2017

What is a sanctuary city?

The term can be a bit vague, but in general, it refers to a city or municipality that refuses to cooperate with federal immigration enforcement.

Some have official policies that prevent law enforcement officials from asking about the immigration status of residents (Los Angeles was the first city to adopt such a policy in 1979), and others refuse to hold undocumented immigrants in jail past their scheduled release dates. None of these policies prevent a police department from pursuing or arresting an undocumented immigrant who commits a crime.

Proponents of these policies say immigration is a federal issue, and that openness over immigration status encourages the undocumented to work with police and report other crimes.

Police chiefs in large cities such as Los Angeles have said that having local cops enforce immigration laws would make it less likely for immigrants to report crimes or engage with the police. Opponents such as the President say that these policies violate federal law and risk releasing criminals.

According to an estimate provided by the New York Times, at least 364 counties nationwide, which includes 39 cities, could be considered sanctuary cities.

What are mayors and politicians in sanctuary cities saying?

This issue will be contentious because so many mayors in the country’s largest cities have expressed their support for immigrants. Rahm Emanuel said Chicago “will always be a sanctuary city.”

D.C. mayor Muriel Bowser issued a statement reading, "The values, laws, and policies of Washington, D.C. did not change on Election Day. We celebrate our diversity and respect all D.C. residents no matter their immigration status." In San Francisco, city attorney Dennis Herrera is investigating whether or not the city can sue the federal government if they end up withholding funding.

And according to New York City’s mayor, Bill de Blasio, “We are not going to sacrifice a half million people who live among us, who are part of our community. We are not going to tear families apart."

Many cities, such as LA, have created legal defense funds to help support immigrants who may be subject to increasing targeting and raids from the Trump administration.

Since Trump signed his executive order this afternoon, many cities and mayors have announced they will continue to function as sanctuary cities, despite the consequences.

What is the potential impact of this executive order?

The potential impact depends on Trump’s willingness to make non-compliance merely an issue with law enforcement budgets, or a stance that triggers a ban of all federal funds for cities that don’t cooperate. Based on the text of the executive order, he may be pushing for the later.

This could not only impact funding for the police, but take away funding for transportation and other big projects. According to an analysis by Mother Jones, cities that continue to be sanctuaries may forfeit billions, depending on the Trump administration’s decisions. Denver, for example, may have $175 million at stake.

There’s also the question of how Trump defines a sanctuary city, and how that definition stands up in court. The Center for Budget and Tax Accountability has noted that to strip all federal funds from sanctuary cities, Trump would likely require Congressional approval of a new law.

The executive order suggests the Secretary of Homeland Security can define a sanctuary jurisdiction, and these jurisdictions “are not eligible to receive Federal grants, except as deemed necessary for law enforcement purposes by the Attorney General or the Secretary.”

What happens next?

Trump has also been unclear in the past about just whom he plans to target, and how many people may be affected.

During an interview with 60 Minutes, Trump said two-to-three million criminals would be deported. Immigration rights advocates dispute that number: Frank Sharry, head of America's Voice, an immigration advocacy group, told the Los Angeles Times that, “We suspect he’s gearing up to implement a very harsh and radical deportation regime and he’s going to deploy it under the pretext they are criminals, when they are not.”

A recent Los Angeles Times article quotes two senior officials in the Trump transition team who said the government may pursue “migrants who have been charged but not convicted, suspected gang members and drug dealers, and people charged with such immigration violations as illegal reentry and overstaying visas, as well as lower-level misdemeanors.”

To accomplish his goals, Trump will have to follow through on previous promises to substantially increase the size of the U.S. Immigration and Customs Enforcement force, which would be necessary if he were to attempt to deport millions of people from the country. The executive order signed today approves the hiring of 10,000 more immigration officers.

Trump’s threats of punishing sanctuary cities may also run afoul of the 10th Amendment, which address the separation of powers between the federal government, states, and cities.

The Supreme Court has ruled that federal government can’t compel local law enforcement to assist with federal law enforcement, hence using funding as a cudgel of sorts to force states or cities to comply (for example, tying federal transportation funding to the drinking age helped push states to adopt a stricter standard).

But a 2012 ruling over Obamacare said that the Federal government can’t be too “coercive,” and take away funds already promised for different reasons. Any move to hold back federal funds would likely face a court challenge.

Here’s the full text of the portion of the order referencing sanctuary cities: