It's that time again in Arlington, an election year when political control of the County Board could be reversed. And although the vote is still eight months away, the race already has begun to take shape.

At stake is the seat now held by board chairman Stephen H. Detwiler, the only member up for reelection this year. But the bigger prize is political power. Detwiler ensured Republican control of the five-member board with his election in 1978, but county Democrats now see the chance to tip the majority in their favor.

Detwiler, 38, already has announced he will seek a second four-year term as a Republican-endorsed independent. Opposing him, by all indications, will be Mary Margaret Whipple, 41, who is not expected to formally announce her candidacy for a few weeks. Candidates won't be officially selected until early May, when Democrats and Republicans are scheduled to meet and discuss their candidates.

Whipple ran unsuccessfully in 1979 as an independent, backed by a coalition of county Democrats and the Arlingtonians for a Better County (ABC), a self-described nonpartisan organization which traditionally endorses Democrats who run as independents.

As independents carrying their party's endorsement rather than nomination, candidates can draw on the active support of the large number of county residents who are federal employes and therefore barred by the Hatch Act from participating in partisan politics.

Arlington's Democratic Committee has gone on record in favor of nominating instead of merely endorsing a candidate at its May 10 mass meeting, but committee chairperson Sharon Davis said she expects the "will of the majority" at the meeting will decide the perennial nonimate/endorse question.

"That means both sides will try to pack the meeting," said Tom Hall, chairman of the ABC, which will hold its own mass meeting sometime after the Democrats. "I'd like to see her run as an independent. I don't think she'll state a preference, but leave it up to the Democrat's mass meeting."

Davis, Hall and others say they don't expect anyone to challenge Whipple because she is very popular and considered to be the strongest candidate to take on the highly popular Detwiler. Jade West, chairman of the Arlington Republican Committee, also says there are no challengers to Detwiler in sight for the party's May 11 "canvass" (primary) because he is perceived by Republicans to be an "odds-on favorite" for reelection.

In Detwiler, Republicans see an opportunity to reverse the trend of the last two board elections, when their candidates were resoundingly defeated by John G. Milliken, who ran as a Democrat in 1980, and Ellen M. Bozman, who ran as a Democrat-backed independent in 1981. The remaining two Republican-endorsed board members--Walter L. Frankland and Dorothy T. Grotos--both come up for reelection next year.

Detwiler, who is serving an unusual second consecutive term as board chairman, and estimates he spends nearly 40 hours a week at it, acknowledges that he might not have run again this year if last year's GOP nominee, Robert E. Harrington, had beaten Bozman. Harrington's defeat was "a major influence" in his decision to run again since the majority would be at stake this year, Detwiler said.

Both Detwiler and Whipple, trading compliments, agree that political control of the board will be a major issue, along with county finances, development along the Metro corridors and "quality of life."

Detwiler says Whipple, who served on the county school board from 1976 to 1980, "would be the most formidable candidate they might run against me. She's attractive, articulate, intelligent and has some recognition from having campaigned previously."

Whipple says of Detwiler, executive vice president of the Continental Federal Savings and Loan Association: "Any time you face an incumbent County Board chairman, you have a difficult race. I think his rhetoric is good and he makes a very good appearance, so I wouldn't underestimate the difficulties of the race."

Early predictions are that the race will be close: Whipple came in third in a four-way race for two seats in 1979, trailing Frankland by 948 votes; Detwiler, whose late father also served on the board, defeated a little-known newcomer by 2,058 votes in a two-way race in his first bid for elective office in 1978.

"A lot of people a while back thought this year was going to be a wash, not worth putting a lot of effort into," said the Democrats' Davis. "But now people feel a lot more enthusiastic and optimistic about our chances. There's a lot of disenchantment out there with the way things have been going."

And Whipple stressed the importance to the Democrats of reinforcing the Bozman and Milliken victories. "Unless a third person is elected to join them this year, their elections aren't going to mean much," she said. "They simply can't do what they're capable of doing without a dependable third person in their majority to join them. At this point, the effective leadership on the board comes from them."

The Republicans, of course, counter with the same argument: They must maintain control to carry out their goals. "I intend to present my incumbency to the voters, and I imagine Whipple will challenge various aspects of it," Detwiler said.