Arlington Democrats and their independent allies, Arlingtonians for a Better County (ABC), have been planning for four years to regain control of the five-member County Board, but until recently few expected the takeover could occur this year.

The reason was County Board Chairman Stephen H. Detwiler, a 39-year-old independent with Republican support whose good looks, moderate record and roots in the community made him seem invulnerable in Tuesday's election.

"He was Mr. Invincible. He was the Robert Redford of the Republican Party," said Arlington Sheriff James Gondles, who had pegged 1983 as the best year for a Democratic/ABC comeback. The two other GOP-controlled seats on the board will be up for election then.

Lately Arlington Democrats have been talking a different game. They've been saying the contest between Detwiler and Democrat Mary Margaret Whipple, a former county school board chairman, is close, "a real horse race," in the words of Democratic Board Member John Milliken.

"In early September, I figured Mary Margaret's chances were pretty slim," echoed Tom Hall, ABC chairman, "but I think it's pretty even right now."

The reasons for the optimism are both local and national. Some, including Gondles, cite a perceived drift away from Republican policies. "In any other year, Steve would be unbeatable," said Gondles.

"This year is a Democratic year, just like two years ago, it was a Republican year," said state Del. James Almand (D-Arlington).

Others say the campaign itself has brought the change. Whipple, who lost a race for the board two years ago, came out charging at her kickoff in early September, with Gov. Charles S. Robb making an unusual appearance on behalf of a local candidate. On that day, 600 Whipple volunteers fanned out across Arlington, dropping off 75,000 leaflets.

Since then, through 24 joint appearances before a kaleidescope of civic groups, she has succeeded in defining the campaign issues, urging a shift away from the Republican majority's policies and putting Detwiler on the defensive with charges about "backroom" dealings. She also has attacked Detwiler on development issues, arguing that Democrats would be more effective at controlling growth along Arlington's gold strip -- the Metro corridor from Rosslyn to Ballston.

Detwiler, a savings and loan company executive whose father was a well-known pediatrician, has reacted strongly to the charges, some say too strongly. "He gets mad easily," said Hall, "He tends to get frustrated."

At each stop on the campaign, Detwiler has defended his record vigorously, noting the drop in the county tax rate since he swung the board into the Republican camp with his election in 1978. He also has defended the board's record on development, particularly the drafting of plans for each of the county's Metro stops.

"My opponent is trying to frighten citizens with images of wall-to-wall concrete from Rosslyn to Ballston," he says. "I think that is misleading. The board has been working very hard to prevent that from happening."

There are other reasons for the unexpectedly close race, Arlington politicians say. Last spring, Detwiler broke with his two Republican colleagues and voted to raise the county tax rate by 2 cents, dampening enthusiasm for his campaign among the county's active and vocal conservative groups.

"He didn't please any taxpayers with that," says Jack Torbet of the Arlington County Taxpayers Association.

Another issue that has been injected into the race is a referendum on creating a housing and redevelopment authority. That has sparked a debate that some say is more impassioned than the Detwiler-Whipple contest.

Detwiler tried to stay clear of the issue, insisting it was separate from his race, but when he came out against the referendum several weeks ago, it helped solidify his support among conservatives, Torbet said.

Whipple has joined many Democrats in supporting the authority proposal that proponents insist would simply provide the county with a mechanism to provide low-interest loans to building owners that want to avoid conversion.

"The Arlington need is a rehabilitation need," said Almand, a chief sponsor of the measure. "This is a method to preserve rental units without using Arlington tax dollars."

Almand objects to any attempt to portray the measure as a prelude to public housing in Arlington. "Clearly, public housing is an erroneous scare tactic," he said, "You couldn't do it if you wanted to and no one wants to."