The National Trust for Historic Preservation is urging the Supreme Court to reject the Trump administration’s request to allow construction of President Donald Trump’s White House ballroom to continue, arguing that the administration cannot bypass Congress to complete the project.

The administration is asking the Supreme Court to put a lower-court ruling on hold while it appeals, arguing that the planned 90,000-square-foot ballroom is part of an integrated national-security facility needed to protect the president and accommodate secure events at the White House. Solicitor General D. John Sauer told the justices that halting construction threatens national security and the safety of Trump, his family and executive branch personnel. The administration also argues that the president has broad authority to make changes to the White House and is challenging the National Trust’s legal standing to bring the lawsuit, contending that Congress did not need to separately authorize the project.

The administration has pointed to security features including an underground military facility, protected infrastructure and other hardened components as evidence that the project goes beyond a conventional ballroom.

The National Trust fired back Tuesday, calling the administration’s effort to keep construction moving while the courts consider the case “astonishing.” “Petitioners, however, insist they will keep building — no matter what anyone says,” lawyers for the National Trust wrote in their response to the administration’s emergency application. The preservation group argues that if the administration believes a new ballroom is necessary, it can ask Congress for authorization rather than proceed without it.

“If Petitioners believe there is a pressing need for a ballroom, nothing prevents them from asking Congress for one — today, tomorrow, or any time in the future,” the Trust said. The group accused the administration of attempting to undermine judicial review and congressional authority by continuing construction while seeking Supreme Court intervention.

The legal fight follows an Aug. 7 ruling by the U.S. Court of Appeals for the D.C. Circuit, which ruled 2-1 that the administration needed congressional authorization for the project. The appeals court’s ruling would halt above-ground construction Friday unless the Supreme Court intervenes, although work on the underground security component has been allowed to continue. The administration has argued that stopping the project now would create a national-security risk and impose significant costs after construction has already progressed substantially.

The Supreme Court filing says the project is already about 65% complete, with roughly 250 workers on site, according to Washington Post reporting. The cost of the project is itself disputed. The administration has continued to put the ballroom’s price at about $400 million, but The Washington Post reported in June that internal documents from Clark Construction put the estimated cost at $600 million, substantially higher than the administration’s public figure. The Post also reported that the internal estimate showed more than half of that projected cost could come from taxpayer-funded sources, despite the administration’s emphasis on private financing for the project. Separately, Reuters has reported that the administration’s broader White House construction program could reach at least $900 million, a figure that encompasses multiple projects and should not be confused with the estimated cost of the ballroom alone.

The National Trust is the plaintiff in the case, but it is not the only organization supporting the legal challenge. The American Institute of Architects and 10 allied architectural, historic-preservation and national-parks organizations have filed an amicus brief supporting the National Trust’s position, arguing that the project raises significant questions involving historic preservation, federal property and presidential authority. The coalition includes the American Society of Landscape Architects, Association for Preservation Technology, Coalition to Protect America’s National Parks, Committee of 100 on the Federal City, Cultural Landscape Foundation, D.C. Preservation League, National Mall Coalition, National Preservation Partners Network, Olmsted Network and Society of Architectural Historians.

The ACLU of the District of Columbia has also filed an amicus brief supporting the National Trust’s position, focusing on separation of powers and the ability of courts to review claims that executive officials have exceeded their legal authority. The Campaign Legal Center and Citizens for Responsibility and Ethics in Washington have separately raised concerns about the project’s financing and potential conflicts of interest. Democratic lawmakers have also joined the opposition, with 143 members of Congress previously filing an amicus brief arguing that Trump lacks authority to undertake the project without congressional authorization.

The administration has maintained that the National Trust lacks standing and that the president has authority to alter the White House. The project involves replacing the White House East Wing with the new ballroom and associated security infrastructure. Trump has repeatedly defended the project, including its security components, while criticizing opponents and arguing that the new facility is necessary to safely host large events at the White House.

The Supreme Court is considering the administration’s request on its emergency docket and could rule before the appeals court’s Friday deadline. If the justices ultimately agree to hear the broader dispute, the National Trust has indicated that it would support an expedited schedule allowing arguments this fall and a final ruling by next summer.