R. Scott Fosler, a seven-year veteran of the Montgomery County Council who was regarded by fellow Democrats as a shoo-in for reelection, said yesterday he will quit politics when his term expires in December to pursue a private career.

The decision by Fosler, 40, to leave the $37,347-a-year council seat is the latest in a series of events to rock Montgomery's Democratic Party, which has been thrown into election-year turmoil by the promise of a bitter primary contest for county executive and wholesale changes in the County Council itself.

Council members David L. Scull and Esther P. Gelman have announced plans to leave Montgomery's seven-member legislative body to run for county executive and Congress this fall, respectively. The Democrats' search for successors to Fosler, Scull and Gelman could play havoc with their tradition of crafting slates of incumbent candidates and could dramatically alter the balance of power on the council for years to come, according to Democratic activists.

"We had a relatively small number of council incumbents running in the first place," said one person active in local Democratic circles. "With Scott leaving, things are opened up even further."

Fosler, a Baltimore native who has lived in Montgomery for 16 years, plans to hold a news conference today to announce his plans. In an interview yesterday, he said he wanted more time to spend with his family and on a private consulting career centered on studies of economic development and state government issues.

"It was a tough choice," said Fosler. "Quite frankly, part of me wanted to stay on."

Fosler's decision to leave elective politics was known by only a handful of friends and political allies, and it struck many associates as in keeping with the independent and sometimes-quirky course he charted through Montgomery's political whirl.

A graduate of Princeton University's Woodrow Wilson School of Public and International Affairs and a professional student of small governments, Fosler, who lives in Chevy Chase, was typical of a generation of Montgomery politicians who used local issues as tickets to higher office.

In the early 1970s, he gained attention as a leader in the civic battles against the commercial development of Chevy Chase and Friendship Heights. In 1978, Fosler launched a campaign to become county executive, but bowed to Democratic front-runner Charles W. Gilchrist and ran instead for a seat on the County Council.

After his election that year, Fosler staked out a reputation as a thoughtful, moderate council member, serving as that body's president in 1980.

However, after the 1982 council elections shifted power to a faction of four Democrats led by Scull and Gelman, Fosler increasingly found himself as a member of the council minority -- a position that some believe was a factor in his decision to quit politics this year.

"Very often, Scott was out there by himself, taking on Gelman and Scull," said one close friend. "I think he felt that eight years of battle was enough."

The division on the council was never starker than in December 1984, when Fosler cast the only vote against the adoption of a 30-year land-use plan that called for a "corridor city" of housing, research and corporate centers in the commuter-clogged wedge between Rockville and Gaithersburg.

"I'm concerned that we may be endangering our own economic health" by saturating the area with new development, Fosler said at the time.

Yesterday, after a year of lively public debate over Montgomery's severe land-use planning problems, Fosler expressed regret over the county's failure to anticipate its record population growth and demand for better public services.

"Planning is not a politically sexy issue," said Fosler, who recently stepped down as president of the Metropolitan Washington Council of Governments, a regional planning agency. "Politicians get concerned with the issues of the moment and that leads to crisis.

"For all the supposed sophistication of Montgomery County, that's exactly what has happened here," he added. "We've lost the lead on strategic planning."

In a year of local election campaigns that may be long on symbols and short on substantive disagreements, an endorsement by Fosler will be coveted by any number of candidates, their spokesmen said yesterday. One of those will be state Sen. Sidney Kramer, who is running for county executive.

"We would view his endorsement as very valuable," said Kramer campaign chairman Lanny Davis, who sources said has raised the issue with Fosler.

Other Democrats believe that Scull also will ask Fosler for an endorsement in the executive race. "If I were David Scull, I'd be on Scott's doorstep tonight," said one party official.

Fosler has made no promises yet to either Kramer and Scull. He said he plans to concentrate on local issues in his final months on the council.

His last project, he added, may be his most important. Fosler has tried for months to establish a commission on Montgomery's future but his council colleagues have not yet dealt with the legislative proposal, he said.