The political story of Montgomery County's District 15 can be told in the events of a recent Saturday when Democratic precinct officials set up a table outside Selby's Market in Poolesville to register new voters: 28 people signed up and half were Republican.

While still in the minority, Republican enrollment is fast gaining in this vast legislative district, and the county GOP, which four years ago established a toehold by winning one of the three House of Delegates seats, hopes to tighten its clasp by capturing at least one more state seat. The increasing number of Republicans in this western and upper part of the county is seen as one of the GOP's best chances for a County Council seat to crack Democratic domination of county government.

"District 15 promises to be a real battleground . . . each side, Democrat and Republican, will get to test their election skills," said Laurence Levitan, the incumbent state senator seeking his fifth term.

Levitan, chairman of the Senate's influential committee on budget and taxation, will face Republican lawyer Robert J. Miller in November but has no primary opposition.

Instead, Sept. 11 will feature crowded House of Delegates primaries by both parties.

A total of 13 candidates -- nine Democrats and four Republicans -- have entered the race, partly attracted by the seat left open when Del. Judith C. Toth, a veteran legislator and formidable Democratic vote getter, decided to run for the County Council.

The two other incumbents, Democrat Gene Counihan and Republican Jean Roesser, are seeking reelection. Counihan, 49, a county school administrator, was first elected in 1982 and has won appointment as co-chairman of a House committee in Annapolis. Counihan is running on a slate with Levitan.

Roesser, 60, a former newspaper reporter, was the only Republican in 1986 to win one of the delegate spots from districts located completely within Montgomery County, placing behind Toth but ahead of Counihan in the overall vote. Roesser has proven to be popular in her district, with even some Democrats praising her as alert to her constituency.

Both incumbents are favored to win their primary challenges. But in each race, the three top vote-getters win and with voter turnout expected to be low in this off-year election, outcomes become more unpredictable because a small number of votes could be decisive. That is especially true in the Democratic primary with its nine candidates.

The other Democrats are: Fernando Bren, 48, an employment consultant and immediate past president of the Montgomery Civic Federation; Wayne Busbice, 61, a former principal at Gaithersburg Junior High School who owns a music company; Joseph P. Foley, 41, a former Capitol Hill aide who now operates a government and public affairs firm; Virginia Gallagher, 74, a retired Capital Hill lobbyist who is active in civic affairs.

Also, Rosemary Glynn, 45, a Bethesda lawyer who ran unsuccesfully for the Democratic nomination in 1988; Sally McGarry, 55, a Democratic Party official who owns a word processing service; Stuart Schooler, 33, a consultant for a commercial real estate broker who has been active in local politics, and Dave Segal, 33, a telemarketing specialist who has been active in environmental issues.

The Republicans, in addition to Roesser, are: Michael J. Baker, 27, an insurance claims adjuster from Damascus; Richard LaVay, 37, a builder who has been active in politics, and William Wilson, 52, a computer specialist and community activist.

The district, which stretches from Potomac to Barnesville, is Montgomery's largest in geographical area. It is the largest in population, because of its tremendous growth since the last redistricting, and one of the more diverse. Among the 75,000 voters are young professionals who moved to Germantown or Montgomery Village because it had housing they could afford, people living and working on family farms and residents with long-term roots in the affluent neighborhoods of Potomac.

District 15 is described by Democratic and Republican activists as the most conservative in Montgomery, and Republicans are within striking distance of Democratic enrollment -- the latest figures show 34,149 Democrats, 29,907 Republicans and 11,121 independents.

The number of Republicans is "growing by leaps and bounds," said Barnesville Democratic Mayor Elizabeth Tolbert, who ascribes the change to an influx of young people who have reached political maturity under Republican presidential administrations. The Democrats, who have controlled local and state government, have also come under criticism by residents, who charge they have lagged in their treatment of the district -- not building the roads or schools to meet the tremendous growth and opting to locate such unpopular facilities as a trash incinerator or a landfill in the district.

The candidates -- tramping door-to-door in generally low-budget campaigns -- point to environmental issues, taxes and transportation as the major concerns of voters. There are some issues -- such as opposition to a western bypass of the Capital Beltway, the need to improve recycling or a north-to-south transit link to the Shady Grove Metro station -- that unify the district and on which the candidates agree.

Other issues, mainly environmental, are more problematic in that the district itself is divided, due in some part to longstanding downcounty-upcounty rivalry. For example, calls for closing the Travilah Quarry in North Potomac worry some residents in Boyds who fear it could mean a transfer of mining operations to their community.

Abortion is also emerging as an issue -- although most candidates see it becoming pivotal in November when the differences between the candidates promise to be more stark. The General Assembly was torn this year by a filibuster over abortion and many predict the pressure will only increase with expectations that the Supreme Court will overturn its decision guaranteeing abortion rights.

On the Democratic side, all the candidates except Gallagher said they favor abortion rights. Gallagher said she would vote to make abortion illegal.

On the Republican side, LaVay said he favors a woman's right to abortion up until the time of viability for the fetus; Baker said he supports a woman's right to an abortion only in the first trimester; and Wilson said he thinks abortion is a decision for a woman to make in conjunction with her beliefs and her doctor's advice.

Roesser said she is personally opposed to abortion, but would not vote to make it illegal in the first trimester. She said she favors restrictions after that except in cases of rape, incest, health of the mother and severe deformities. She has voted to restrict public funding for abortions, and she abstained on a bill designed to prevent abortion protesters from blocking access to medical facilities because, she said, the situation was adequately covered under existing law.

"It will be a big issue when we get to confront Jean Roesser with her record," said McGarry. Toth, the district's top vote getter, opposes abortion.

Meanwhile, the candidates -- particularly the Democrats -- are struggling to set themselves apart to voters who are largely indifferent to what the state legislature does.

Counihan is stressing his experience, arguing that Montgomery will need proven representatives in the next session when a state commission issues its recommendations on revising Maryland's tax structure. Foley is building his base around the influx of new voters in North Potomac and he hopes his opposition to the Travilah Quarry will strike a responsive chord. Busbice, championing the preservation of agricultural lands and green space, has been embraced by upcounty leaders, including Mayor Tolbert, who think they will get best representation from someone who lives where they do.

Gallagher stresses her work as a civic activist to better manage development and to slow growth. Bren is criticizing the quality of the county's representation in Annapolis and promises to do a better job of winning state aid. Glynn is striking hard on the abortion issue and pointing to her endorsement by groups such as the teachers union and the National Organization for Women. Segal is focusing on the environment, saying more needs to be done in recycling to spare rural Montgomery from landfills and incinerators. Schooler said he is running on a record commited to fiscal responsibility and sees the major issues as transporation, drugs and education. McGarry, who is married to county Transportation Director Robert McGarry, points to her 20 years of working and living in the community and is emphasizing the environment, transportation, education and the quality of life.

Meanwhile, the Republicans are striking themes that criticize Democratic fiscal and growth policies as well as the ability of Democrats to bring state aid to the county.

Wilson was one of the first candidates to seize on Montgomery's taxpayers revolt, adopting the theme of a "taxbuster" and vowing to push for a constitutional amendement to cap property taxes. LaVay says the county is in a mess because the Democrats have not done a good job of getting state money for needed services. Baker agrees that Montgomery taxpayers shouldn't be paying for the rest of the state, particularly when it is lacking roads and schools. Roesser pointed to what she called a successful first term and promised to continue work on transportation and environmental issues and for fuller public disclosure of campaign finances.

The GOP, in short, has its eye on November.