Montgomery County Council member Esther P. Gelman and former U.S. representative Carlton R. Sickles yesterday joined the already crowded field of Democratic candidates seeking the party's nomination to Maryland's 8th District congressional seat.

Gelman, 54, a longtime member of the council, and Sickles, 64, who served in Congress in the mid-1960s, formally opened their campaigns at outdoor news conferences before small groups of supporters.

Both stressed their long careers in government -- Gelman her 16 years on the county planning board and council, Sickles his tenure as a state delegate and his two terms in Congress. And they portrayed themselves as vigorous opponents of many of the domestic and foreign policy initiatives of President Reagan.

They join three Democrats who are hoping to succeed Rep. Michael D. Barnes, who is making a bid for the U.S. Senate. State Sen. Stewart Bainum Jr. and lobbyists Leon G. Billings and Wendell M. Holloway also have announced for Barnes' seat.

Republicans Constance A. Morella, a state delegate from Bethesda, and William S. Shepard, a retired Foreign Service officer, are vying for the GOP nomination in the heavily Democratic district, which includes much of Montgomery.

With more than five months before the primary election, the Democratic race in the 8th District is shaping up as one of the most hotly contested in the state. Sickles, Gelman and the other candidates -- each of whom plans to spend at least $200,000 in the race -- are hoping to expand their distinct bases of support into coalitions that will deliver victory in the September primary.

For instance, Sickles, a lawyer who has strong ties to the national labor movement and who was a member and chairman of the Metro board during the turbulent formative years of the regional transit system, said he needs to become better known among Montgomery's young voters.

"I'm telling them it was my father who was in Congress before," he joked yesterday.

Meanwhile, Gelman, who has long been active in women's groups and Jewish organizations, has begun to soften some of the rhetoric that has drawn barbs from her council colleagues and County Executive Charles W. Gilchrist, a longtime political antagonist.

Gelman said yesterday that winning the primary would turn on "the perception of who can operate best in the tough arena" on Capitol Hill. "When you know your county like I do, you make a much more effective member of Congress," she added.

Gelman addressed about 30 supporters on a noisy street corner in Bethesda near the National Institutes of Health, a federal research facility that she said was unfairly targeted for a budget cut by the Reagan administration. Gelman attacked many of the president's efforts, saying the thrust of his programs was "bounded spiritually by Wall Street on one side and by Rodeo Drive on another."

Sickles, speaking to about 60 friends in a parking lot off busy Georgia Avenue in Silver Spring, emphasized the need for continued federal funding of Metro subway construction and more stringent U.S. regulation of savings and loan associations.

The candidate entertained the crowd with a spirited, if off-key, harmonica rendition of an old Irish folk song.

County Council member Neal Potter, who has tangled frequently with Gelman over the years, appeared at Sickles' gathering but told a reporter he favors Billings in the primary race.

"Leon's ahead at the moment," Potter said. But, he added, "Either he or Carlton would make an excellent congressman. The key to this race is knowing the ropes and handling them well."