Six months after local budget autonomy advocates celebrated their voter-approved Budget Autonomy Act surviving a congressional review period unscathed, the Government Accountability Office has declared the law null.

In a legal opinion released Thursday, the GAO concludes that provisions amending the budget process laid out in the Home Rule Act have “no legal effect” and Congress maintains its constitutional power over the District’s purse.

Supporters of the budget autonomy law reject the congressional agency’s conclusion, saying the city is still on firm constitutional and legal footing to proceed with its implementation.

GAO’s opinion is “just that — just an opinion,” said DC Vote Executive Director Kimberly Perry.

The GAO’s stance echoes that of Republicans on the House Appropriations Committee, who issued a report, endorsed by Speaker John A. Boehner, R-Ohio, that declared the budget autonomy referendum approved by 83 percent of D.C. voters in April 2013 “an expression of the opinion of the residents, only.”

Florida Republican Ander Crenshaw, who holds the top spot on the House Appropriations panel that oversees D.C.’s annual appropriation, requested the GAO report this summer, days after the D.C. Council-passed bill cleared the 35-day congressional review period with no objection.

Perry said the GAO is merely a “political arm of Congress,” and the law can only be reversed through an act of Congress or a court’s decision to overturn it.

Del. Eleanor Holmes Norton, D-D.C., who has previously expressed skepticism about the constitutional and legal standing of a referendum, released a statement vowing to continue defending it in Congress.

“Notwithstanding legal and political questions that have been raised, I will continue to work to head off any congressional efforts to block or overturn the referendum or to penalize the District for pursuing the referendum,” Norton said. “While GAO opinions are certainly well respected in Congress, they have no legal effect. What is most clear is the need, desire and momentum for congressional action for full budget autonomy.”

Despite GAO’s opinion, D.C. Council Chairman Phil Mendelson maintains the law “is right, legal, and proper” and said in a statement that the new budget procedure meets the requirements of the Antideficiency Act.

“Given the ambiguity caused by GAO, Congress should take this opportunity to acknowledge explicitly the right of District residents to determine how their tax dollars should be spent,” he said.

As written, the law unlinks the local budget from the congressional review process that’s been in place for three decades, allows District officials to set their own fiscal calendar and spares city government the anxiety of future budget showdowns and resulting shutdowns, like the one that gripped the nation for 16 days in October.

Unlike a budget autonomy bill advanced by House Oversight and Government Reform Chairman Darrell Issa, R-Calif., the law also removes the threat of policy riders, routinely added by Republicans who use control of the purse strings to steer social policy in the heavily Democratic city.

GAO cites the Antideficiency Act, which prohibits District officers and employees from obligating or expending funds except in accordance with appropriations enacted by Congress. It also says D.C. remains bound by the Budget and Accounting Act, which requires D.C. to submit budget estimates to the president.

President Barack Obama and Senate appropriators both endorsed language in their 2014 budget proposals that would have provided for budget autonomy. That language did not make it into the final spending deal for fiscal 2014.

The GAO’s opinion expresses no view on the merits of budget autonomy, instead finding “it is a matter that rests with the Congress.”