Montgomery County's Democratic candidates, sitting on a 2-to-1 voter registration edge, are counting on a strong appeal to partisanship, a perceived dislike for Reaganomics, and a national mood favoring incumbents to coast them to victory on Nov. 2.

Republicans, who earlier this year boasted of making major inroads because of the Democrats' primary bickering, now seem content to talk publicly about winning an overly optimistic maximum of seven seats (out of 18) in the House of Delegates, three (out of six) in the State Senate and two on the seven-member County Council. Republicans think maybe, with a large Republican voter turnout and a last-minute surge among undecideds, they can capture the county executive's office.

But privately, Republican candidates and party officials say they are fighting to hold on to their current three seats in the House and their lone Senate seat, of Howard A. Denis, the moderate from District 16 who is facing an aggressive young challenger, Brian Frosh. Denis is buoyed by a spate of endorsements and his telephone poll, but Frosh -- who has been trying to link Denis to President Reagan -- thinks "this is going to be a strong Democratic year."

The GOP's best shot at gaining a Senate seat appears to be in District 15, which includes Potomac and all of the upper county. Sen. Laurence Levitan, chairman of the powerful budget and taxation committee, is being challenged by the state's recently reelected GOP party chairman, Dr. Allan C. Levey. In District 17, including Rockville and Gaithersburg, Rockville City Council member Phyllis B. Fordham is waging a strong campaign against Democratic incumbent Sen. S. Frank Shore.

Republicans, who hoped to capitalize on the disunity that wracked the Democrats before the primary, said they have been surprised at how quickly the majority party has pulled back together, in appearance if not in fact. Also, they concede, the Democrats -- led by Gov. Harry Hughes, Sen. Paul Sarbanes, Attorney General Stephen H. Sachs and Comptroller Louis L. Goldstein -- have managed to exploit "the Reagan factor," blaming the GOP for the current economic woes.

Also, some GOP candidates and party regulars said they were disappointed that the candidates at the top of their local ticket, banker Joseph C. McGrath for county executive and Elizabeth W. Spencer for Congress, have waged such lackluster, low-visibility campaigns. Both have been criticized privately in their own party for failing to develop specific themes or offer clear alternatives to the Democratic incumbents.

After concentrating their energy on winning the primary over better known GOP opponents, McGrath and Spencer were left with just six weeks to increase their low name recognition, build a case against the incumbents, and convert Democrats.

McGrath has railed against Gilchrist's administration as "an embarrassment," and he has promised to "restore integrity" and bring a banker's management experience to the job. But said one GOP leader, who is supporting McGrath, "His theme is Gilchrist's integrity, and I don't think it's worked. It just never took off. And Gilchrist gets him on his lack of experience."

One strategist for moderate GOP candidates compared the McGrath and Spencer postprimary campaigns to the last scene of the movie "The Candidate," where Robert Redford, basking in his election victory, says to his campaign mastermind, "What do we do now?"

On the County Council level, the GOP put its best hopes on two moderates, Leonard Robinson, an international development specialist who is running at-large, and Alvin Jones Arnett, a former director of the Appalachian Regional Commission. Both reportedly have received quiet support and advice from one of the Democrats' two feuding factions. Those rumors escalated when Arnett showed up at a fund-raiser for Democratic Council member Esther P. Gelman, de facto leader of one of the Council factions.

Even so, Arnett concedes that he is a long shot against incumbent Scott Fosler in the 1st District. "I am not optimistic, I am realistic," he said. Robinson may have a better chance since his incumbent Democratic opponents, Council President Neal Potter and David L. Scull, led opposing slates in the primary. Voters can choose two at-large candidates. The Robinson victory scenario has Scull supporters voting for Scull and Robinson, while Potter supporters vote for Potter and Robinson.

The Republicans are basing their hopes now on a large GOP voter turnout. They are assuming that Democratic voters won't bother to vote, since polls show Democratic incumbents in state-wide races sitting on wide margins. To turn out the GOP vote, the county's Republican Party for the first time is operating a full-time phone bank staffed with paid workers.