Montgomery County voters decided last night that the growth and development that have made the county an affluent success story were threatening its future, and turned to a man who promised to control it.

Neal Potter, 75-year-old veteran of Montgomery political and civic life, won the Democratic nomination last night in his party's primary, and his against-the-odds defeat of incumbent County Executive Sidney Kramer was widely seen as a protest against large-scale development, traffic gridlock and crowded schools.

"It means that the 1980s are over in Montgomery County," said Montgomery Planning Board Chairman Gus Bauman. "People are very nervous about what the future will be in the 1990s. The nineties will be a lot slower."

The forces that propelled Potter's campaign -- discontent with rapid growth, distrust of the competence of county officials to handle it, a desire for change -- are part of a growing chorus of citizen frustration in Washington and its suburbs.

Voters in Fairfax County were the first to feel the effects of the explosive growth that transformed their neighborhoods in the 1970s and '80s. In a race that presaged yesterday's vote in Montgomery, slow-growth advocate Audrey Moore shocked the political establishment with her upset of incumbent Fairfax County Board of Supervisors Chairman John F. Herrity.

In the District, voters rejected the mayoral candidate most closely tied to the development community, John Ray.

Indeed, in 1990, a campaign contribution from a developer was often as much a liability as an asset.

"I'm scared," said James Goeden, an official with the Bethesda-Chevy Chase Chamber of Commerce, who in the days before the primary sensed that results would bring bad news for developers. "I think I'll get my ticket to Frederick."

"I have just had enough," said Silver Spring resident Donna Anderson, moments after she cast her vote for Potter.

Potter's win -- making him a favorite to win the Nov. 6 election to head Maryland's largest jurisdiction -- portends profound changes in both the style and substance of county business, and how Montgomery relates to other parts of the state.

Last night, as shell-shocked state and county officials gathered at Kramer's campaign headquarters in Silver Spring to commiserate, they looked ahead to an administration led by a man who says candidly that he cares more about ideas than administration, a man who is more comfortable at a dinner listening to recordings of birdcalls than at a power lunch.

"Is it an upset? You bet it is," Potter told a jubilant crowd of supporters after Kramer called Potter to concede defeat. "We do not have a lot of single-issue interests controlling this county. The people of this county can control their own interests."

State's Attorney Andrew L. Sonner, a Potter supporter, pointed out that Potter overcame Kramer's huge advantage in campaign contributions and newspaper endorsements to win.

"He had to take on spending 10 to 1. He had to take on The Washington Post . . . . He happened to be right on the majority of issues. It's a new day."

"The way the government will change will be very substantial. You cannot overstate it," said Jacqueline Rogers, a former county budget director who now serves in Gov. William Donald Schaefer's Cabinet.

Obervers see a massive change in department heads, a change in many policies and an undoing of some decisions.

It appeared last night that Potter -- who for much of his 20-year political career was a maverick in county government -- will have a council friendly to his ideas for a tax on development or a $120 fee on employee parking spots. Five Democratic council candidates endorsed by Potter -- incumbents Bruce T. Adams and Isiah Leggett and challengers Gail Ewing, Marilyn Praisner and Derick Berlage -- appeared to be headed for victory.

It appeared that incumbent Rose Crenca, whose support of a massive redevelopment in downtown Silver Spring antagonized many residents, and incumbent Michael L. Gudis, who received a large percentage of donations from developers, lost their races. Other Democratic winners appeared to be incumbents Michael L. Subin and William E. Hanna Jr. and political newcomer Vickie York.

"It sends a strong message that there is a strong mandate for policies consistent with . . . a need to slow growth down a little bit and get our finances in order," Leggett said.

"Forget about the trolley," said Montgomery Transportation Director Robert S. McGarry, referring to plans to build a light rail system from Bethesda to Silver Spring with $70 million in promised state funding.

Potter has said he favors a delay in the project. Potter also has been an opponent of developer Lloyd Moore's regional shopping for downtown.

In the days leading up to yesterday's election, Potter stressed that while he saw his administration making changes, he would bring his studied, cautious approach to government.

In the end, perhaps more than anything else, it was Potter himself -- his reputation as the conscience of the county, Montgomery's elder statesman -- that made the difference in yesterday's balloting.

Despite efforts by the Kramer campaign to pin the blame for overdevelopment on the years Potter served on the council, voters just would not buy the image -- that a man whose parents died months after their family farm was overtaken by development was now supporting it.

And if yesterday's vote was an acceptance of Potter, it was a pointed rejection of Kramer, a successful businesssman who built a fortune from a chain of car washes.

Kramer often appears unemotional, and chooses his words so carefully that he sometimes seems calculating and cold. In the days leading up to the primary, Kramer was clearly frustrated that while polls showed Montgomery residents happy and satisifed with county life, they didn't appear willing to give government -- and specifically him -- any credit.

Yesterday at Kemp Mill Elementary School -- Kramer's old home precinct -- Cindy Gerstl showed up to vote, and after wrestling over recent days she decided not to make a selection for executive.

"I wasn't happy with either," she said, and offered her opinion that it really didn't matter who was executive.

Yesterday's very low turnout -- fewer than half of the eligible Democrats in a county of more than 750,000 residents -- suggests that others felt the same.