Douglas M. Duncan strode door to door. It was early evening. Time to come home, start dinner, read the mail and greet the kids.

One hand clasped red, white and blue campaign literature. The other was free to shake hands, to write a personal note, to slip a brochure under a doormat.

The routine was a familiar one. Born and bred in Rockville, Duncan traces his political activism before his three City Council election victories to childhood when he tagged along on neighborhood "lit drops" with his mother.

Now, at age 32 a political veteran, Duncan is running hard to become mayor of his lifelong home town, but this latest campaign is more than a race: It's a crusade.

"Rockville is at risk" is his warning. "Let Rockville be Rockville" is his promise.

According to Duncan, the traditions and values of Rockville are under siege from the style and policies of Mayor Steven Van Grack. Duncan, on the stump and in interviews, charges that the progressive city government that invited citizens to participate in decision-making and that helped make Rockville a four-time All-American city is wilting under Van Grack.

"There is something very special about Rockville, and I, for one, don't want to see it changed," said Duncan, a staff manager for AT&T, a husband and a father of three boys.

Duncan's style was reserved but earnest as he explained how he knew in February that he had to run when the mayor and a majority of the council voted to renew the contract of City Manager Richard V. Robinson.

"That's what got me; I knew that this man was all wrong for the city and that it was obvious the mayor was relying on him too much. I knew I had to run," said Duncan, who has made Robinson's ouster a plank in his mayoral platform.

The result, five days before the city's 24,000 eligible voters can go to the polls to elect a mayor and four council members, is a particularly bitter and, by all accounts, close race. Most attention has centered on the battle over the mayor, who sits on the council and has the same vote as any single council member but has the power to set the agenda.

Duncan has effectively put the incumbent on the defensive on such issues as the amount and source of his campaign contributions.

Rockville politics are strictly nonpartisan and traditionally have been low-key affairs fueled by volunteer help. Van Grack's ability to raise larger sums of money and his use of a sophisticated political consultant have come under fire from Duncan, who said a victory by his opponent will change the face of the city's grass-roots politics.

However, Duncan's choice of tactics has provided fuel for his critics, who see him as a politician whose ambition has made him ruthless.

"I view Douglas as a footprint candidate. His footprints appear all over the backs of people," said Stephen N. Abrams, the city's senior council member, who is running for reelection on Van Grack's slate.

Abrams cited Duncan's attack on the city manager as going to the heart of the government. "I am not saying you shouldn't fire someone if it is merited, but you don't make a political issue out of it," Abrams said. " . . . What kind of reasoned professional would want to come work for a government where his or her job is dependent on the whims and caprices of elected officials every two years?"

But what Abrams views as Duncan's ruthlessness, the candidate's supporters hail as evidence of his uncompromising integrity. "He calls them the way he sees them," said Alexander Greene, a former Rockville mayor who is an honorary cochairman of Duncan's campaign.

Duncan's celebrated break with former mayor John Freeland provides perhaps the best example of this contradictory image.

When Duncan became the youngest council member ever elected, at age 26 in 1982, it was in part through the political help of the Independents for Rockville, and its standard-bearer Freeland. Duncan split with both after Freeland accepted a job with a development company doing business with the city and the IFR refused to come out against the situation.

Duncan was outspoken and pushed strongly for reform of city ethics laws. Freeland resigned amid the flap, and today is supporting Van Grack in his relection bid.

Blair Lee, the county's former lobbyist in Annapolis and the former campaign manager for gubernatorial candidate Stephen H. Sachs, said he initially had suspected Duncan was grandstanding on the issue and would use it to run for mayor. "When he didn't run {for mayor in 1985}, he impressed me as a different kind of politician . . . someone who did something that was not politically smart and made him a lot of enemies but something he thought was right," Lee said.

Abrams has a different opinion: "He stepped all over John Freeland, and wrongly so, just to enhance his reputation."

Supporters also point to Duncan's decision last year not to accept the inducement of Democratic Party regulars to run for the Montgomery County Council on a slate headed by Sidney Kramer. Several sources said that Duncan turned down what would have been a sure shot for higher political office because it would have pushed off council member William E. Hanna Jr., a former Rockville mayor whom Duncan considers a family friend.

"There are some things not worth the loss of friendship," Duncan said.

Van Grack, too, is considered a friend, Duncan said. "He's a nice guy, I like him," he said of the incumbent, but indicated that the difference is the job he is doing and its importance to the city's future.

Duncan said he thinks his Rockville roots prepare him for the role of mayor. One of 13 children, Duncan grew up in the Twinbrook neighborhood, going to St. Mary's School in Rockville and then St. John's College High School in the District. He went to college at Columbia University. Duncan's father, James T. Duncan, worked for the National Security Agency. It was his mother Eleanor who got him hooked on politics. She remembers his first campaign. "It was 1956 and Adlai Stevenson. Douglas was my baby . . . in the carriage," as she passed out campaign literature, she explained.

Duncan worked in Charles W. Gilchrist's 1978 county executive campaign, and after the successful race he got a job with the County Criminal Justice Coordinating Commission. He left government service, he said, after he married his wife Barbara and they had children. "We needed more money," he said of his decision to work for the phone company.

Serving on the City Council, he said, has satisfied his itch for politics and he thinks has helped him repay the city for his good life.

Duncan, who has raised $11,554 for the race, has promised to relieve congestion on Rockville Pike and to protect neighborhoods from uncontrolled growth. He sees affordable day care as a need, opposes rent control and wants to improve county-city relations. Running with Duncan on his slate are incumbent council member James F. Coyle and hopefuls David Robbins, the former county recreation director, and Viola D. Hovsepian, a former mayor. The mayor is paid $8,000 a year.