The Arlingtonians for a Better Country (ABC), one of the oldest and largest independent political reform movements on the East Coast, waged its toughest election battle in nearly a decade last week and lost, a victim of the kind of Proposition 13-style backlash that has won widespread national acceptance.

At stake in the hard-fought county board race between Republican-backed independent Stephen H. Detwiler and Joseph N. Pelton, an independent endorsed by the ABC and the local Democratic party, was political control, of the five-member county board which, since 1969, has been controlled by the ABC coalition.

For years, Republican-backed candidates have hammered unsuccessfully at "ABC control" while Arlingtonians elected ABC candidates, often by wide margins.

Election results show that Pelton's 2,000 vote loss hinged on two large bellweather precincts in the wealthiest section of North Arlington. Those precincts went decisively for Detwiler, who made property taxes the campaign issue, despite his opponent's frequent reminders that Arlington has among the lowest tax rates in the metropolitan area.

"Taxes are absolutely the issue in those precincts," said school board Chairman Ann Broder, a 20-year veteran of ABC. "I can't tell you how many huge homes I sat in with Cadillacs parked in front and women who'd just emerged from the beauty parlor within the last 24 hours who actually told me they just couldn't afford to pay their property taxes."

"My feeling is that there was a strong negative vote against the ABC," said Pelton. "People had a clear idea of what they were voting against."

What they were voting against was a political organization whose 600 members, according to chairman Helen Weyant, are typically "well-educated, professional, politically liberal, very conscientious" and, despite professions of nonpartisanship, virtually indistinguishable from liberal Democrats.

ABC has been blasted and envied by Republicans as good government machine capable of mobilizing hundreds of loyal party workers to do the unglamorous, unpaid "grunt work" that is the backbone of successul political campaigns. ABC's sole involvement is the yearly county board race; the organization is active between the May convention, at which a candidate is endorsed, and the November election.

The issues that generally concern ABC are as traditionally suburban as Arlington was when the group was founded in 1955: taxes, schools, services and an intangible, but often cited, "quality of life" theme.

ABC was founded as an alternative to the segregationist Byrd machine-which then controlled the Democratic Party-by young, ambitious federal employes barred by the Hatch Act from partisan campaigns. For all but three years in the late '60s, ABC controlled a majority of county board seats.

"Everytime we've lost control, people start talking about the demise of ABC," said County Board Chairman John W. Purdy. "You hear the expression that we've just a bunch of tired liberals, but we've come back from a lot lower defeats than this."

"I think the ABC has gotten tired," said Kenneth Haggerty, a prominent Arlington Republican and a former County Board member. "But this is not their swan song. We had a very appealing candidate who ran a pretty intelligent campaign."

Other observers say ABC was beaten at its own game. "The Republican precinct organization was just 500 percent better than it's been," said Broder.

In the past, Republican candidates have not mounted strong, well-financed grassroots campaigns and have been perceived as reactionary, a liability Detwiler, a 35-year-old bank vice president, managed to avoid.

There also was the Detwiler name, an asset the winner, a third-generation Arlingtonian, acknowledges. Son of the late Dr. Robert H. Detwiler, a former County Board member who, according to one ABCer, was "everybody's beloved pediatrician." Detwiler noted, "Some people told me they voted for me out of love for Dad."

Others point out that Pelton was forced to run on the ABC record-which included present property tax rates-but was unable to personally claim credit for any of its accomplishments.

"It could be that Joe just didn't turn voters on enough," said ABC Chairman Weyant, echoing others who faulted Pelton's earnest, rather plodding style and, for weeks, said privately they felt he was a weak candidate.

Next year is equally crucial for the Republicans because the terms of both Dorothy T. Grotos and Walter L. Frankland Jr. expire.

"When you pick up a majority," cautioned one high-ranking Republican close to Frankland, "you can't act like Attila the Hun and purge all the citizen's committees and everything or you just turn on the opposition."

"It's a lot easier to run when you're out of power," Weyant agreed. "We'll all have had a year to see what Walter Frankland does about Metro and how the people of Arlington react."