New York City Mayor Bill de Blasio (D) wants the federal government to reimburse the city for the costs of protecting President-elect Donald Trump’s residence and transition headquarters. He cites the particular challenges of securing a “highly-trafficked, dense urban environment, and one that presents an unprecedented and unique target for potential terrorist activity.”

De Blasio requested $35 million to pay for the police stationed at Trump Tower in a Monday letter addressed to President Barack Obama and another to Congress. That sum would cover the period from Election Day to Trump’s inauguration.

“Locating a residence within New York City that requires ongoing Presidential-level security presents unprecedented law enforcement concerns. Trump Tower is located in the heart of a prime commercial area in Manhattan,” the mayor wrote. “It is a high-density neighborhood and street traffic easily obstructs pathways to and from the building making it profoundly challenging for the NYPD to establish a secure perimeter.”

Mayor de Blasio is sending a letter to Obama formally asking for reimbursement for Trump Tower security pic.twitter.com/g4VHKLzUnp — Noah Hurowitz (@NoahHurowitz) December 5, 2016

Trump Tower has long been Trump’s home base. Since his election last month, he has spent the vast majority of his time there, conducting meetings for Cabinet and staff picks and otherwise using it as a headquarters for his transition team. While Trump and his family receive Secret Service protection, the city’s police also assist in the security effort, particularly in protecting the building from the throngs of onlookers.

City officials estimated that security around the building is costing city taxpayers as much as $1 million per day.

“While the United States Secret Service provides an unparalleled level of personal security to the President-elect and his family, the NYPD is responsible not only for their safety, but for the security of thousands of Trump Tower residents, employees and visitors, and hundreds of thousands of people who are in Midtown at any given moment,” de Blasio wrote.

Neville Elder/Corbis via Getty Images New York police officers keep watch outside Trump Tower in midtown Manhattan on Nov. 18, 2016.

During a press conference on Monday, de Blasio said he would be requesting meetings with the Obama administration and members of Congress to go over the matter in detail. He said that he had already spoken to Steven Mnuchin, Trump’s pick for Treasury secretary, to request “a real important discussion to be had on how we deal with reimbursement issues going forward.”

The city’s costly responsibility will not end on Inauguration Day. Trump’s wife Melania and son Barron are expected to remain in New York until the end of the school year at least, and the president-elect has indicated that he will frequently return to Trump Tower even after moving to the White House.

De Blasio ― who vocally opposed Trump’s candidacy, calling him “a blowhard” and “dangerous” ― met with the president-elect at Trump Tower a week after the election. He described the meeting as cordial, but said he told Trump that “sending a message of unity is crucial” and “tried to express to him how much fear there is” regarding some of Trump’s most prominent campaign proposals, including banning Muslim immigration and deporting 11 million undocumented immigrants.

“We need to give assurance that the rights of all New Yorkers and all Americans will be respected,” de Blasio said.