The contest for the Republican nomination for Montgomery County executive has turned into a nip-and-tuck struggle among three strong candidates arguing over who is most experienced, most loyal to the GOP and best able to attract the Democratic votes needed to win in November.

The three agree the race will be decided in the 10 days remaining before the Sept. 14 primary. Up to now the campaign has been largely invisible, and the candidates are concentrating on getting their supporters out to the polls since the race could be won with as few as 8,000 Republican primary votes.

State Del. Luiz R. Simmons, a moderate-to-liberal Republican who has represented the Rockville area in the General Assembly since 1979, is relying on one final mass mailing and his daily three-hour appearances in front of Magruder's supermarket in Rockville -- Montgomery County's version of the factory gate.

Bank vice president Joseph McGrath, a business-oriented, more conservative party worker who has lined up support from GOP regulars, has begun a radio advertising campaign that touts his long re'sume': Gonzaga High School, Harvard, Army lieutenant and vice president of American Security Bank. McGrath, together with his slate of seven County Council candidates, is distributing his final batch of literature by walking door-to-door in key precincts daily and by standing in front of the Silver Spring Metro station some mornings.

Simmons and McGrath say the gruelling daily personal appearances help increase name recognition for the contest after the primary.

But real estate man John P. (Jack) Hewitt, the former executive director of the Maryland-National Capital Park and Planning Commission and conservative Democrat who switched to the GOP in May, says the personal appearances by his opponents are a waste of time since the Republican primary is decided by only a handful of voters. He is concentrating on one last mass mailing to 34,000 Republican households and on word-of-mouth contact through party leaders.

All three candidates plan to operate telephone banks in the final week.

The primary race is representative of the county Republican Party's continuing civil war between moderates who have traditionally controlled Republican elections and the long-isolated hard core of conservative party elite. The latter have never won an election but are heartened because Ronald Reagan carried the county with a plurality in 1980.

Conservatives recruited both Hewitt and McGrath to challenge the more moderate Simmons.

A recent poll commissioned by County Executive Charles W. Gilchrist, the Democratic incumbent, showed all three Republicans bunched tightly together with Simmons slightly ahead, according to pollster Keith Haller of Potomac Survey Research, who did not release the actual figures. Some Republican observers, discounting the findings of that poll, called the GOP race a two-way struggle between the best-funded candidates, McGrath and Simmons -- a view both candidates accept.

At forums and debates before elderly groups, tenants and various Republican clubs, the three candidates are trying to picture themselves as the most experienced, the most electable, and the most loyal to a party that is outnumbered 2 to 1 on the county registration rolls and now has practically no party organization.

Simmons reminds audiences that he is the only one of the three to win elected office. He begins the answer to almost every question with the phrase, "In the General Assembly, I . . . " He then launches into a list of legislative accomplishments.

McGrath, who has never run for office before, boasts of his banking experience and calls himself a "public servant, not a professional politician." He calls himself "a consensus Republican" since his supporters span the political spectrum from conservatives like GOP Central Committee member Carole Plante to Republican moderates like lawyer Tony Morella and unsuccessful 1978 executive candidate R. Maxwell Keeney.

Hewitt calls himself the only one in the GOP field with executive experience -- from his directorship of the powerful planning commission -- and with any experience at all in county government. He asked a senior citizen audience of about 50 "to consider a candidate with the administrative ability to run the executive branch of government."

Each candidate also is fighting some negative perceptions. Simmons is trying to persuade conservative Republicans that he is not a closet Democrat. McGrath is trying to persuade voters that he is not a complete unknown with no past in the county. Hewitt, meanwhile, is battling a perception that he really doesn't want the job.

Simmons was elected to the legislature in 1978 by putting together his own organization in Rockville, and some GOP regulars have accused him of shunning the official organization ever since. Said Plante, McGrath's campaign manager: "He snubs his nose at the Republican Party. He's running for Lou Simmons and that's all he cares about. And as somebody who's worked in the Republican Party, I resent it."

The party's small but vocal conservative faction also resents Simmons' positions on some issues, which they consider too liberal, particularly his advocacy of increased taxes on local country clubs.

Simmons moved to counter those accusations at a candidates' forum before the Montgomery County Republican Club. He said the question for executive candidates should be, "How can we build the Republican Party in this county for the next four years? Many of our Republican leaders, once they get in office, put a great deal of distance between themselves and the party. I believe in a strong party because it helps all of us."

Simmons starts with an edge in name identification. In front of Magrauder's supermarket in Rockville, he was recognized by about half of the 50 people who took his literature. Some offered comments like, "I've read about you. You're a Mathias-style Republican."

McGrath, as a newcomer, tried early to establish credibility by proving he could raise money. He claimed that he would raise $100,000 for the primary, and have $50,000 of that in the bank by the first filing deadline last month. In the end, McGrath raised only $37,000, and $15,000 of that was money he lent to himself.

McGrath has recently been spending about two hours late every afternoon in a Bethesda office, telephoning his network of Gonzaga High School graduates, Harvard alumni, and banking associates, peddling tickets to a $50 fund-raiser.

He is also fighting for name recognition. "We're coming from zero," Plante said in front of the Metro stop. "We've got to keep our visibility up." To help out, McGrath ran a series of radio advertisements in Ocean City and Annapolis during the last week of August. "That last week of August, Republicans are going to be on vacation," McGrath explained.

Hewitt is fighting the perception that his campaign is low-key, which cast doubts on whether the ex-Democrat is running for the job, or out of a grudge against some of his former fellow Democrats. At a recent candidate forum, Simmons entered the meeting room, shook hands with each of the 30 persons there and handed each a piece of his literature. Hewitt, in contrast, walked straight to the front and took a seat at the head table.

Hewitt said he is concentrating on the Republican primary now. "Simmons gave out a brochure to every person out there," Hewitt said. "Hell, there probably weren't three Republicans in the room! How the hell are they going to help him in the primary?"

"Everybody says I'm running a low-key campaign," said Hewitt. "But the public isn't excited about this election. It's just a dull election. What the hell am I supposed to do except be low-key when nobody's excited about this thing?"