Leon G. Billings, a veteran Capitol Hill staff member, opened his campaign yesterday for Maryland's 8th District seat in the U.S. House with an unabashedly liberal agenda and verbal barbs for two fellow Democrats.

Flanked by his wife, three children and former senator Edmund S. Muskie (D-Maine), his former boss, Billings told a crowd of 100 supporters in Rockville that he is the most qualified candidate in the crowded field of Montgomery County Democrats vying to succeed Rep. Michael D. Barnes, who is running for the U.S. Senate.

"I won't need on-the-job training," said Billings, who is now a lobbyist. "None of my opponents bring to this race the experience I have."

Maryland state Sen. Stewart Bainum Jr., County Council member Esther P. Gelman, Ford Motor Co. lobbyist Wendell M. Holloway and former representative Carlton R. Sickles also have announced their intention to seek the 8th District seat, which encompasses much of the county.

Republican Constance A. Morella, who represents Bethesda in the House of Delegates, is seeking the Republican nomination. The Democratic Party enjoys roughly a 2-to-1 advantage in registered voters over Republicans in the district.

Billings, 48, who played a key role in the drafting of Congress' landmark environmental legislation in the late 1960s and early 1970s, pledged to fight for antipollution laws, congressional support for Israel, an arms reduction agreement with the Soviet Union, human rights and the interests of federal employes, thousands of whom live in the district.

"Montgomery County has been a place of hope and opportunity," Billings said in a brief address to the enthusiastic audience. "Whether this continues will depend in large part on the decisions Congress makes in the next few years . . . . "

Bainum, a millionaire from Silver Spring, and Gelman, a skilled politician from Potomac, have taken the early lead in organizing and fund raising for the Democrats' Sept. 9 primary, but Billings characterized them yesterday as out of touch with the voters in the generally affluent district.

Billings attacked as "a particularly bad idea" Bainum's proposal last year to have state government make a 2 percent cut in the number of its management employes. "It speaks volumes about how he might vote in 1987 on Reagan budget cuts," said Billings. "Fortunately, his proposal failed."

Billings also sharply criticized Gelman for opposing major reductions in the U.S. defense budget.

"That sounds an awful lot like Ronald Reagan to me," said Billings. "Does Esther Gelman really think there is no fat in a $300 billion Defense Department budget, no unnecessary weapon systems, no redundant military expenditures?"

Billings, a red-headed Montana native who spices his down-home style with a tart tongue, boasted yesterday of his family's strong ties to local civic organizations and Montgomery Democrats. Billings has lived in Silver Spring since 1967 and coached soccer for 10 seasons; his wife Patricia has been a member of the county Democratic Central Committee since 1974.

Muskie, for whom Billings worked nearly 15 years, said his former protege would be a worthy successor to Barnes.

"Leon's instincts are right," said Muskie. "He's pro-environment, and at a time when the role of the federal government is shrinking, still believes government has responsibility for things other than defense, foreign policy and paying the interest on the national debt."

Billings now spends time on Capitol Hill as a coal-tax lobbyist for Montana Gov. Ted Schwinden. He felt some political heat recently when legislators in Helena, the state capital, wondered whether he could lobby and campaign for Congress at the same time.

Billings tried to placate the politicians by telling a reporter at the Montana statehouse that he would campaign mostly on weekends and at night.

Yesterday, though, Billings sounded more aggressive than that. "Wait till you see my 91-year-old grandmother come back and campaign for me," he said.