A bathroom sign welcomes both genders at the Cacao Cinnamon coffee shop in Durham, North Carolina May 3, 2016. REUTERS/Jonathan Drake/File Photo

Republican leaders in North Carolina on Thursday refused to back down from a law regulating which restrooms can be used by transgender people after the federal government told the state the law violated the U.S. Civil Rights Act.

The state’s leading Republican lawmaker, House Speaker Tim Moore, told reporters in Raleigh that North Carolina would not be “bullied” into meeting the U.S. Justice Department’s Monday deadline to change the law, know as HB2, or House Bill 2.

North Carolina is the first U.S. state to pass a law requiring transgender people to use public bathrooms that correspond with the sex on their birth certificate instead of the gender with which they identify.

Republican legislators in several other states have proposed similar laws, making transgender rights a hot-button social issue in an election year.

The Justice Department’s challenge is similarly unprecedented, though the agency has in the past intervened on behalf of transgender individuals who have alleged discrimination.

The Republican National Committee urged state legislators in January to resist federal policies that allow transgender people to use restrooms of their choice at public schools.

The Justice Department said in letters to North Carolina Governor Pat McCrory and other state officials on Wednesday that the state had until Monday to decided if it would stop discriminating against transgender state employees.

McCrory affirmed his support for the “very common-sense rule” before a group of business leaders on Wednesday night but said he did not know if the state would fight the Justice Department. His office did not respond to questions on Thursday.

If McCrory does not stand down from enforcing HB2, the Justice Department’s civil rights division could push for a court order. If a federal judge sides with the agency, North Carolina would have to comply or face a reduction in federal funding.

On Thursday, Republican Senate leader Phil Berger’s spokeswoman, Amy Auth, said state lawyers were reviewing the letter. Berger on Wednesday said the department’s position represented “a gross overreach.”

North Carolina’s Secretary of Public Safety and officials at the University of North Carolina also received similar letters on Wednesday.

Margaret Spellings, president of the University of North Carolina system, said her office would respond to the department’s letter by the deadline.

“We take this determination seriously and will be conferring with the Governor’s Office, legislative leaders, and counsel about next steps,” she said.

Justice Department spokeswoman Dena Iverson would not say whether the department would take legal action against the state.

The bathroom provision in HB2 is being challenged in federal court by critics including the American Civil Liberties Union and Lambda Legal, which advocates for lesbian, gay, bisexual and transgender (LGBT) rights.

“We’ve never before seen a state do something as aggressive as require mandatory discrimination against its own transgender employees,” said Peter Renn, an attorney with Lambda Legal, in a phone interview with Reuters.

Jillian Weiss, an attorney who has argued in federal court for transgender rights, said the Justice Department’s action against North Carolina showed the federal government was willing to intervene when states passed laws reducing rights based on gender identity and sexual orientation.

Weiss predicts Mississippi, where a law allowing businesses to refuse service to gay people takes effect in July, will be the department’s next target.

(Additional reporting by Julia Edwards and Julia Harte in Washington; Editing by Alistair Bell and Alan Crosby)