Incumbency seemed to be the greatest political asset in Montgomery County's Democratic primary last week, as voters consistently rejected challengers' calls for new blood, fresh ideas and a more open process.

In the end, the controversial "incumbents' slate," called Montgomery Democrats For '82, swept 55 out of 58 offices, including party central committee seats, all but two legislative seats and the courthouse posts.

In the hotly-contested County Council primary, with two Democratic slates running against each other, only one incumbent, Ruth Spector, lost.

On the Republican side, voters seemed to be sending much the same message of pragmatism and moderation. Republican voters were cruel to their established early "front-runners," Marian Greenblatt for Congress and Luiz R. Simmons for county executive. Instead, they opted for two low-profile, consensus candidates: Elizabeth Spencer for Congress and Joseph C. McGrath for county executive.

Greenblatt and Simmons were flashier and began their races with higher name recognition, but their outspokenness alienated some segments of the GOP. Many moderates found the conservative Greenblatt an embarrassment after her campaign-launching attacks calling Rep. Michael D. Barnes a PLO supporter. The more liberal Simmons alienated conservatives, who thought he was a closet Democrat, particularly for his proposal to increase taxes on country clubs.

"The average Republican voter is very much like the average Democratic voter," said GOP state Sen. Howard A. Denis, who was unopposed for renomination. "What they're saying is that they appreciate steadiness, consensus-building, conciliation and moderation. They want candidates who are nonthreatening. The same people who voted for Elizabeth probably also voted for Joe McGrath."

Edmond F. Rovner, special assistant to County Executive Charles W. Gilchrist, likened the voters' call for moderation in the partisan primaries to the signal they sent in the nonpartisan school board race. In that race, moderates opposed to the current board's conservative policies led the field of eight finalists. Voters in November will select four from that group.

"It's the same thread running through the school board race," Rovner said. "People were upset by negativism and name-calling."

When the incumbents first decided this summer to band together on a slate of convenience, there were howls of "bossism" and "undemocratic!" from challengers and dissidents locked off the slate.

One challenger was Tony Fisher, a 32-year-old black county police officer who waged an energetic and well-organized challenge to incumbent Sheriff James A. Young. Fisher, in a highly visible campaign that caught the attention of the press for a traditionally invisible office, portrayed Young as a bad administrator who mismanaged $200,000 in legal fees.

But Young had the advantage of incumbency, and the incumbents' slate. When the absentee votes were counted, the sheriff won by 308 votes out of 60,000 cast.

"We were using the Fisher/Young race as the bellwether to see how well we would do," said Victor L. Crawford, the outgoing state senator who served as chairman of the incumbents' slate. "I knew if we could get Jim Young through, we could get anybody through."

The only place where the incumbents' slate fell apart was in the 16th Legislative District. The two incumbent delegates, Nancy Kopp and Marilyn Goldwater, were renominated, but their handpicked choices for the state Senate and for the third delegate seat lost to slate "outsiders" who were running against "the machine."

In the County Council race, incumbents on the current all-Democratic council split into two camps, "The Merit Team" of Esther Gelman, David Scull and Michael Gudis, and the opposing "United Democrats:" Council President Neal Potter, Rose Crenca, Scott Fosler and Ruth Spector. Each of the two warring camps filled out its seven-member slates with nonincumbent challengers.

But voters rejected straight slate-voting and split their choices between the two. Spector, the only losing incumbent, was beaten by William Hanna, Rockville's caustic and colorful four-term mayor.

"There were no real surprises," said outgoing Democratic party chairman Stanton J. Gildenhorn. "I think we will have real party unity now."

Gildenhorn and other Democratic party leaders were expressing some glee at the Republican results. They had not relished the thought of running a general election campaign against either Greenblatt or Simmons, whom they saw as aggressive campaigners, constantly on the attack.

Sizing up the coming race between Spencer and Democratic U.S. Rep. Michael D. Barnes, Gildenhorn said: "We will now see a congressional race on the issues, between a person who believes in Democratic principles and a person who believes in Republican. There will be no name-calling. There will be no demagoguery."

Rovner similarly sized up the coming battle between Gilchrist and McGrath, a relative unknown who espouses traditional Republican values. "There'll be less debate on the facts and more on the policy implications," he said. "It is likely to be a more mannerly campaign."