Steve Armstrong's conversion came in 1981. That was the year he switched from Democrat to Republican, two years after he worked for the reelection of Fairfax Democratic Supervisor James M. Scott.

Armstrong, 50, a retired Marine and a lawyer, now is seeking to oust his old ally from his position as the Providence District's representative on the Fairfax County board, an office Scott has held for 12 years.

But first Armstrong has to defeat another lawyer, John D. Austin Jr., 36, for the Republican nomination for Scott's seat. Voters in the district will decide Tuesday which man will carry the GOP banner this fall against Scott in what promises to be a three-way race with independent William M. Lockwood, former head of the county planning commission.

Providence District, one of Fairfax's most densely populated districts, straddles Fairfax City and includes Merrifield, Mantua and Oakton. It contains numerous older, busy roads, including Rte. 29 (Lee Highway) and Rte. 50 (Arlington Boulevard), and many houses and apartment complexes built in the 1950s and 1960s. The median family income of $30,232 is below the county average, of $33,236.

Scott's advocacy of housing for poor and moderate-income families has made him a controversial figure. He won his last race by less than 600 votes and some Republicans are saying that he is equally vulnerable this year.

Armstrong says that the seeds of his discontent were sown during the 1979 campaign. He says he then defended, on Scott's behalf, a proposal by the county housing authority to place subsidized housing units in Jefferson Village, an aging apartment complex on Rte. 50. Armstrong says he was led to think that only a small number of units would be subsidized, but in 1981 he says he discovered he was wrong. Armstrong went to court to stop the housing authority and got the agency to change its plans. He also decided to switch parties. "I found that the only Democrat taking arms against those sea of troubles was me," Armstrong said yesterday.

Austin, vice president of the American Mining Congress, is not impressed with Armstrong's party credentials. "I've been a Republican for a long time and he's a new Republican. I have the support of the party."

Armstrong replies that he decided to run in the primary because "several Republicans asked me to run." Armstrong says Austin's support is from the Republican Party faction that has been at odds with Ben Partin, the conservative chairman of the Fairfax GOP. Austin acknowledges that he has sided with the moderate faction of the party.

To accentuate the contrast, Armstrong wrote, in campaign literature that went out recently: "I am the only Conservative Republican candidate running . . . ."

This is the first bid at elective office for both, although Armstrong opposed Ann H. Kahn in 1980 for the Providence District seat on the school board, whose members are appointed by supervisors. Scott chose Kahn.

Armstrong, who spent 20 years in the Marines, has picked red and yellow as his campaign colors. "They're Marine colors. I'm grateful I wasn't in the Army," says Armstrong. "Camouflage colors wouldn't go over very well in Fairfax County."

Both candidates say the campaign has been low keyed, although Armstrong says he expects it to "heat up in the closing days." Armstrong says he thinks it will be close. Austin does not.

"I haven't run against any supporters or workers for him Armstrong ," says Austin. "Either they don't know him or they don't even know there is a primary race."