A group of 31 mayors on Thursday released an open letter asking President Obama to extend protections for undocumented immigrants, as he enters the final weeks of his presidency.

Writing under the umbrella of the nationwide organization Cities for Action, the mayors, including NYC Mayor Bill de Blasio, asked the Obama Administration to fast-track applications for the Deferred Action for Childhood Arrivals program, which offers a temporary shield from deportation to certain undocumented immigrants who came to the United States as children. They also recommended expediting applications for Temporary Protected Status, a special designation for immigrants from countries deemed unsafe to return to, and asked Obama to consider expanded the designation to cover more countries.

The mayors further called upon Obama to institute strict privacy measures for participants in DACA, many of whom now fear that information they provided to federal immigration authorities when they registered may be used by the coming Trump Administration to identify and deport them.

Obama has been frequently criticized by immigrants' rights groups for the scale of deportations that have been carried out on his watch. He's presided over the deportation of more than 2.5 million undocumented immigrants, the most of any president.

But with the most openly anti-immigrant presidential candidate in recent memory poised to take office, the mayors struck a conciliatory tone, thanking Obama for policies like DACA, as well as the decision last week to dismantle the Bush-era National Security Entry-Exit Registration System, which required men from Muslim-majority countries to register with immigration authorities.

"As mayors and county executives in the Cities for Action coalition, we write to thank you for your leadership on behalf of immigrant families and urge you to take action to ensure continued support for vulnerable immigrants in our communities before you leave office," the signatories wrote.

Since Donald Trump's election, Mayor de Blasio has promised to shield New York's large undocumented population—estimated at 500,000—from any attempts by the Trump administration to deport New Yorkers en masse. New York already has policies on the books that prohibit city agencies from cooperating with federal immigration authorities, but de Blasio has said he will take further steps if necessary to protect the city's undocumented. He is currently embroiled in a lawsuit filed by several Republican lawmakers over plans to scrub vital identifying information about undocumented immigrants from databases for the city's municipal ID program, ID NYC.

"Our immigrant residents help make New York City stronger economically, culturally, and socially. We have benefited from President Obama's actions to protect vulnerable immigrants during his time in office," de Blasio said in a press release accompanying the Cities for Action open letter. "In the wake of the election, many of our residents are uncertain about the future. That's why I am standing with my fellow mayors to thank the President for his leadership and call on him to take additional, lawful steps to help immigrants before leaving office."

In addition to de Blasio, mayors of many of the nation's largest cities signed the letter, including Chicago Mayor Rahm Emanuel—who delivered a letter in support of DACA to Trump earlier this month—and L.A. Mayor Eric Garcetti, as well as the mayors of Philadelphia, San Francisco, Boston, and Denver.

Donna Lieberman, executive director of the New York Civil Liberties Union, issued a statement in support of the letter's message. "Donald Trump poses an unprecedented threat to civil rights, civil liberties, and democratic values," she wrote. "Resistance is vital, and mayors have an important role to play—combating hate, protecting public dissent, supporting sanctuary cities, protecting privacy, ending discriminatory police practices, and more. Now is the time for elected officials to show backbone."