D.C. Council Chairman Phil Mendelson (D) announces the council's lawsuit against the District's mayor and chief financial officer Thursday. He is flanked to the left by lawyers Brian Netter and Karen L. Dunn, who are representing the council pro bono. (Mike DeBonis/The Washington Post)

The D.C. Council sued the mayor for the first time in more than a decade Thursday, challenging Vincent C. Gray’s refusal to abide by a ballot measure granting the city greater fiscal freedom from Congress, even as Gray emphasized his sympathy with the council’s ultimate goal.

Council Chairman Phil Mendelson (D) announced the lawsuit outside the D.C. Superior Courthouse moments after it was filed. “We have no choice,” he said.

The council moved in 2012 to put the issue of budgetary autonomy — the subject of much unsuccessful congressional lobbying — in front of voters, testing a novel legal theory that maintains that the city charter can be amended to change the city’s financial relationship with Congress.

Under the referendum, about $7 billion in local tax revenue could be allocated by the council annually without a separate congressional appropriation, allowing the city to operate during federal shutdowns and put its fiscal year on a different calendar than the federal government’s.

The ballot measure passed last April with 83 percent support. But a series of legal opinions from D.C. Attorney General Irvin B. Nathan and the congressional Government Accountability Office held that it had “no legal force or effect” and that only an act of Congress could change the city’s budget process.

Gray (D) and Chief Financial Officer Jeffrey S. DeWitt told Mendelson in separate letters last week that they would defy the legislature’s attempts to implement budget autonomy. Mendelson said those letters forced the council’s hand.

On Wednesday, Gray said that he welcomed the lawsuit, saying he was “looking forward to there being a solution.” He said he thought it was unlikely, however, that the council’s arguments will prevail.

“I am a huge proponent for budget autonomy,” Gray said. “I want to do it in what I would call a lawful way. . . . I think ultimately it is going to have to be Congress that changes it.”

Mendelson said he is confident that the courts will uphold the referendum as valid. “We feel that we have a very good case here,” he said. “This is not something frivolous.”

The lawsuit says Gray’s and DeWitt’s positions undermine the council’s lawmaking authority and would “impede the orderly administration of the District government.” The council’s pro bono attorneys argue that Congress gave the District full control of its financial accounts when it granted home rule in 1973.

It is unclear what role, if any, Congress might take as the lawsuit proceeds. A spokesman for Rep. Darrell Issa (R-Calif.), the House Oversight and Government Reform Committee chairman, declined to comment. Issa has supported legislative efforts to secure budget autonomy but has expressed doubts about the referendum’s validity. Del. Eleanor Holmes Norton (D) said she will work to prevent any attempts to overturn the referendum on Capitol Hill.

Outside the courthouse Thursday, the leader of a key D.C. voting-rights group expressed frustration that the branches of District government — even as they espoused common goals — were fighting among themselves.

“We thought we’d be challenged by Congress,” said Kimberly Perry, executive director of D.C. Vote. “We never thought in a million years we’d be challenged by our own executive [branch].”

An initial hearing is set for July 18 in D.C. Superior Court. The council has requested expedited consideration so that the case can be resolved before late May, which is when the 2015 city budget normally would be due to the White House.

Any review of the lower court’s findings would go to the D.C. Court of Appeals, the District’s highest local court, whose judges are presidentially appointed and confirmed by the Senate.

Aaron C. Davis contributed to this report.