Montgomery County state Sen. Stewart Bainum Jr., wrapping himself securely in the philosophical mantle of fellow Democrat Michael D. Barnes, launched a campaign yesterday to succeed Barnes in Maryland's 8th District House of Representatives seat.

Bainum, the 39-year-old heir to one of the largest fortunes in the Washington area and an unsuccessful candidate for Congress in 1981, repeatedly invoked the name and politics of the locally popular Barnes in a speech before 300 supporters in Silver Spring.

"We now expect our congressman to be a local leader with a national voice," Bainum said in a reference to Barnes, who is leaving the House to run for the U.S. Senate. "I will give you that leadership."

Bainum, who was planning last summer to run for the Senate until Barnes announced for the seat, is one of six Montgomery Democrats who intend to run for the House in the party's September primary.

The other Democrats are private consultant Bernard Aronson, former Capitol Hill staff member Leon G. Billings, County Council member Esther P. Gelman, Ford Motor Co. lobbyist Wendell M. Holloway and former representative Carlton R. Sickles.

Two Republicans, Montgomery Del. Constance A. Morella and retired Foreign Service officer William S. Shepard, have announced for the 8th District seat.

Bainum leads his Democratic and Republican rivals in fund raising for the election, having collected $52,338 in contributions last year, according to reports filed Friday with the Federal Elections Commission. Bainum has told supporters he needs to raise at least $250,000 for the primary, which many experts believe will be the most costly Democratic election in Montgomery history.

"It's going to be quite a race," said Democrat Carol Petzold, one of a dozen candidates for political office who was working the crowd at Bainum's rally.

"I don't think these [congressional] candidates differ a great deal on the issues," added Petzold, who is running for the Maryland House of Delegates from District 19. "The race is one of perception."

A champion of liberal causes, including his highly publicized fight to take away a state tax break from the all-male Burning Tree Club, Bainum has a decidedly conservative streak on financial issues.

For example, in the 1985 session of the General Assembly, he infuriated the Senate leadership by waging a doomed battle to cut the number of managers in state government by 2 percent.

More recently, in a special legislative session in October, Bainum railed against the proposed sale of Merritt Commercial Savings & Loan Association to a New York financial institution because of the multimillion-dollar windfall the sale provided to the thrift's owner.

"Stewart's a sharp guy who knows what he's doing," said Marilyn Piety, a civic leader in Silver Spring who attended the rally. "He always wages an impressive campaign. I'm sure this one will be no different."

Bainum launched his campaign yesterday from the Armory building in downtown Silver Spring, not far from his native Takoma Park and the corporate headquarters of Manor Care Inc., the giant health care firm that has generated a family fortune estimated at well over $100 million.

Bainum pledged to fight in Congress for antihunger programs, a fairer tax structure and an end "to gold-plated weapon systems like the MX missile, which do not increase our defense." Bainum called a bilateral nuclear freeze between the United States and Soviet Union "the most serious responsibility of our generation."

"Montgomery County is entitled to national leadership, national dignity," said Gilbert B. Lessenco, a Democratic Party elder and Barnes strategist who endorsed Bainum yesterday. "Stewart offers us that opportunity."