Members of the Arlington County Board's new Democratic majority said yesterday they plan to take a much tougher line toward developers and will be less anxious to cut services, especially in the schools, than was the Republican board majority that was defeated in this week's elections.

Those are some of the changes board member-elect Mary Margaret Whipple and her two Democratic colleagues said residents can expect in January after she joins the five-member board.

Neither Whipple nor the other Democrats would predict yesterday if the new board would be able to hold property taxes at their current levels, but all three said in separate interviews that they believed some county programs have been hurt in the past by cuts demanded by the board's Republican majority.

"The budget is the toughest thing we're going to be facing next year," said Whipple, a former school board chairman who defeated incumbent Republican-backed independent Stephen H. Detwiler Tuesday by about 4,000 votes. Whipple said she views her election as a "ratification of the priorities the Democrats have had." Those priorities, she said, include more careful examination of the school budget, recreation programs and developers' proposals.

The three Democrats also said they are committed to working with County Manager Larry J. Brown, a fiscal conservative whom the board named earlier this year to run the county bureaucracy. Brown had been directed by the current board to prepare a budget for the coming year based on continuing real estate taxes at the current rate of 98 cents per $100 assessed value.

The Democrats said it was too soon to predict if they would approve a budget at that rate. State and federal aid to the county is dropping and the county's overall property assessments, they noted, are expected to grow at a rate of only 1 to 3 percent this year. Assuming minimal growth in county spending, officials already are projecting a budget shortfall of $6.7 million next year at the current tax rates.

"I'd like to see what the trade-offs might be, what kind of sacrifices we might have to make to change that rate," Democrat John G. Milliken said.

The Democrats said they expect to establish clearer lines of communication within the county government and with county residents. "We will try to articulate a clearer sense of directions and goals for the county," board member Ellen M. Bozman said. "I think everybody has been getting mixed signals in the past few years, which has been very hard on everyone, including the County Board, the staff and the citizens."

"Yes, people want development, and so do I," Whipple said. "But it is crucial that development not overwhelm residential neighborhoods" along the two Metro subway corridors in the county.

Whipple's election also will enable the Democrats to change political control of the five-member county School Board. All five members of that board were appointed by the Republican majority, but three of those seats will be up for reappointment within 18 months.

Along with the other Democrats, Whipple expressed concerned about the direction of the school system, contending that the school board has too often "sacrificed excellence to small economies which were eroding the quality of the schools."

"There was certainly a message from the voters that the schools remain a very high priority and that message was lost sight of a bit" under the Republicans, Milliken said. "But I don't sense a mandate to change the fundamental policies and direction of the schools. It seems to be more a question of priorities and resources."

Arlington Republicans blamed Detwiler's loss on anti-Republican sentiment and poor precinct work. They also expressed considerable surprise that Detwiler, a savings and loan executive, was not able to capitalize on his opposition to a proposed redevelopment and housing authority, which the voters rejected overwhelmingly.

"My analysis convinces me that Steve took on his shoulders the anti- Reagan administration feeling of many Arlingtonians," said County Board member Walter L. Frankland, a member of the GOP majority. "As a result, it displaced local issues completely. This election was not determined by local issues," said Frankland who, with board member Dorothy T. Grotos, will form the minority on the board. Both of their seats are up for reelection next year.

Frankland predicted that county taxes and spending would increase under the Democrats and added: "I need to hear more from the people to see if they know what they did to themselves and, if they really want that, they deserve it."

"Before a Republican-endorsed candidate or a Republican is going to be elected or reelected, we're going to have to change some of our priorities and start concentrating more on electing county candidates than we have in the past," said Robert E. Harrington, Detwiler's campaign manager.