Since he was announced the victor of the 2016 election, Donald Trump has said he would immediately deport 3 million undocumented immigrants as president. But for many Democrats, that kind of mass deportation would be inhumane and harmful to communities across the nation. 

School board officials and Democratic mayors from major U.S. cities have vowed in recent days to fight any federal plans to deport undocumented immigrants, despite Trump’s vow to withhold millions of dollars in taxpayer money if they do not cooperate. New York City’s Bill de Blasio, Chicago’s Rahm Emanuel and Seattle’s Ed Murray are among several Democratic leaders of so-called “sanctuary cities” attempting to ease the qualms of millions of immigrants left in the dark about their futures under a Trump presidency, NBC News reported Wednesday.

"Since the presidential election, there has been a sense of uncertainty among many immigrant communities in Chicago and across the nation," Emanuel said at a news conference Monday, according to CNN. "I want to assure all of our families that Chicago is and will remain a sanctuary city."

Los Angeles Police Chief Charlie Beck said he would not change the LAPD’s decade-long stance to not uphold federal immigration laws, the Los Angeles Times reported. Los Angeles Mayor Eric Garcetti agreed with the policy, explaining that crime victims and witnesses might be deterred from talking to law enforcement authorities if they have fears they might be deported.

The Los Angeles Board of Education also voted Tuesday to maintain two current polices aimed at keeping local public schools a “safe zones” for immigrants and their children. One policy barred Federal Immigration and Customs Enforcement officials from entering school campuses without the prior consent of the superintendent and the district’s lawyers. The other policy protected the immigration information and identities of students, family members and school staff. Board members wrote a joint letter to Trump, “affirming the American ideals that are celebrated in Los Angeles.”

Mayor Jorge Elosa of Providence, Rhode Island, the son of Guatemalan immigrants, also said he wouldn't halt the city’s policy of refusing to hold people charged with civil infractions for federal immigration officials.

There is no legal definition for the term “sanctuary city.” But it is generally regarded as a place where local police don't coordinate with U.S. Immigration and Customs Enforcement officials working to deport undocumented workers, according to the Washington Post.

Cities like San Francisco, Los Angeles and New York City had been viewed historically as a safe havens for immigrates. They all have implemented extensive programs on providing resources to immigrants, such as issuing local ID cards allowing them access to government or other municipal services. 

Philadelphia Mayor Kim Kenny restored sanctuary status when he took office in January and said last week the city would act against federal attempts to deport its residents. District of Columbia Mayor Muriel Bowser said in a statement Monday Washington would remain a "sanctuary city" and would do what it could to protect immigrants once Trump takes office, according to local reports

During a campaign speech in Phoenix in September, Trump promised his supporters that he would “end the sanctuary cities” by blocking federal funds to whichever city chose not to cooperate with his deportation plans.