A cache of data on undocumented immigrants accumulated by New York City will not be penetrated without a “real fight,” Mayor Bill de Blasio pledged.

In January 2015, the city launched IDNYC, the country’s “most ambitious municipal identification program,” according to the office of the mayor. The program provided more than 850,000 people, many who are undocumented immigrants, a free and legitimate identification card without having to disclose their legal status.

The card could be used to open bank and credit union accounts, apply for a library card, get prescription discounts through BigAppleRx, and provide access to cultural attractions in New York.

The card also provides trans people a way to verify their gender identity.

“This card represents who we are: New Yorkers who value equality, opportunity and diversity,” de Blasio said at the program’s unveiling.

In light of President-elect Donald Trump’s professed desire to track down undocumented persons, many aspects of the city’s ID program are being questioned and present new challenges to the city in privacy protection. Trump reiterated his pledge to pursue “illegal” immigrants on his post-election 60 Minutes interview which aired Sunday.

But the mayor resolved to prevent the future president from accessing the city's encrypted IDNYC databases, which contain reams of personal information—addresses, contact information, birth dates and places of origin. And it cannot be disclosed to federal law enforcement or immigration authorities without permission from the city’s human resources administration, CNN reported.

"[Trump] can change some federal laws but the Constitution protects a lot of the rights and powers of localities," de Blasio said.

The concern is that the city’s database could end up as the polestar of Trump’s deportation endeavor.

"So on something like [the ID database], I think because it touches that button directly of whether people's personal privacy is going to be respected,” de Blasio said in a news conference. “I think that's one where there would be a real fight."

The mayor also said he might activate a provision of the law to destroy the information: A part of the ID law permits information to be erased by the end of the year, a deadline reportedly included because of the prospect that a “Tea Party Republican” might be elected, a proponent of legislation told CNN.

"As you know there's been an ongoing plan regardless of any electoral activity how long records are kept. Given this new reality we're certainly going to assess how we should handle it," he said.

"I want everyone in New York City to know that we are standing by our values and we will fight to protect our values," de Blasio said.

In addition to the seizing of information, New York City’s status as a “sanctuary city,” which in some cases can protect law-breaking undocumented persons from deportation, is also vulnerable. Trump has mentioned an intent to remove funding for sanctuary cities.

"Block funding for sanctuary cities ... no more funding. We will end the sanctuary cities that have resulted in so many needless deaths," Trump said at a campaign stop in Phoenix in August. "Cities that refuse to cooperate with federal authorities will not receive taxpayer dollars.”