When Arlington voters go to the polls Tuesday to pick a new County Board member, they will also be making a host of crucial decisions on Metro, taxes, schools and the structure of county government.

Should Republican-endorsed candidate Stephen H. Detwiler defeat Joseph N. Pelton, who is backed by a liberal coalition of Democrats and the nonpartisan Arlingtonians for a Better County, political power on the five-member board would, for the first time in a decade, shift to the Republicans.

Both candidates and party leaders say a Republican victory is a real possibility.

A victory for Detwiler, observers say, could mean major changes in a county that has strongly supported Metro and last week voted to submit and estimated $50 million bond referendum to the voters next year to help finance completion of the full 101-mile system.

Presently three members of the board - Chairman John W. Purdy, Vice Chairman Ellen M. Bozman and outgoing member Joseph S. Wholey, who is also chairman of the regional Metro board - represent the ABC coalition, which has held a majority of seats on the board for most of the past 25 years. Board members Dorothy T. Grotos and Walter L. Frankland Jr. are endorsed by the Republican Party.

County Board candidates run as independents with party endorsements rather than as Republican or Democratic nominees because of the Hatch Act's ban on partisan political activity by the many federal workers in Arlington.

Neither Pelton nor Detwiler, both 35, has the benefits of incumbency, an advantage ABC candidates have enjoyed for most of the last 10 years. Wholey, who earlier this year decided against running for a third term, was wildly regarded as virtually unbeatable.

Leaders of both parties, assessing Detwiler's prospects, noted that for the first time in recent memory the Republicans have copied the successful precinct organization that is the bulwark of ABC's strategy. Detwiler also has equaled Pelton in fund raising.

Others point to Detwiler's cleancut, blond good looks and speculate that a voter backlash in the year of Proposition 13 would help the GOP-endorsed nominee.

(A third candidate, Richard Gardiner, 26, a lawyer for the National Rifle Association, is running with what he calls a "largely symbolic" endorsement by the Libertarian Party. Gardiner favors slashing property taxes 30 percent by charging for traditionally free county services like trash collection. He also favors halting the Metro system.)

"I'm confident of victory," said Frankland, vociferous critic of Metro who opposes completing the system. There is widespread speculation that a Detwiler victory would mean that Frankland would become chairman of the County Board or Arlington's delegate to the Metro or both.

Last spring Detwiler, son of a former conservative County Board member, said that if elected he did not intend to be "a duplicate voice" for Grotos or Frankland. But both have regularly appeared with him at candidate's nights, are prominently pictured on his campaign literature and frequently mentioned in his speeches.

"Make no mistake," said one high-ranking Republican official, who asked not to be identified. "If he's elected, Steve will do whatever Walter wants him to do. After all, Walter convinced him to run in the first place."

Like Pelton, Detwiler has said he favors completing the full Metro system. However, noted the official, Frankland might favor holding up money for Metro or reversing the recent decision on the bond referendum.

A vice president of First Federal Savings and Loan of Arlington, Detwiler has said he would institute "Proposition 6 1/2" whereby the rise in real estate assessments would be offset by a corresponding reduction in the property tax rate.

Also included in Detwiler's plan is a selective hiring freeze to reduce employment in county government through attrition. Last September the board defeat a similar proposal by Frankland.

Pelton, a South Arlingtonian, is executive assistant to the director of the International Telecommunication Satellite Organization.

He is running on the ABC platform, which says that Arlington has among the lowest property tax rates in the area as well as good schools and services because of the ABC. Pelton also has released a tax plan under which tax bills would not increase more than the rate of inflation.

"The effect of Proposition 13 is the great unknown in this election," Pelton acknowledged. "It's irresponsible for my opponent to say he supports Metro, schools and a tax reduction. He's going to have to make enormous cuts in services."

Pelton also disputes Detwiler's repeated accusations that the "ABC machine" is responsible for a decline in quality in Arlington schools. "He keeps saying that the school system is going to wrack and ruin, but our SAT scores are higher than Fairfax and Alexandria."

Both Pelton and Detwiler have raised about $25,000 each, according to financial reports.

Also running in Arlington are three candidates for a one-year term as commissioner of revenue. They are Clyde Boden, 57; Leo Urbanske, 55, who is running as an independent with Republican endorsement, and Democrat Geraldine Whiting, 49.