Many Democrats snickered publicly and many Republicans were privately skeptical when Republican Stephen H. Detwiler vowed he would not be "a duplicate voice" for either Dorothy T. Grotos or Walter L. Frankland Jr., the senior Republicans on the Arlington County Board.

But in the 15 months the 36-year-old bank officer has been on the board, Detwiler increasingly has shown that he meant what he said.

Last Saturday, for example, as the Republican-dominated five-member board was haggling over the size of a real estate tax cut, Frankland, the board's chairman and the man who plucked Detwiler from political obscurity and persuaded him to run for office, proposed a 20-cent cut in the tax rate.

Frankland argued that a reduction of that size would offset a steep rise in assessments and thus fulfill one of his campaign promises -- to compensate for higher property values with corresponding reductions in the tax rate.

When Grotos said she intended to vote with Frankland, Detwiler said a tax cut of the size Frankland recommended would mean delaying long-term, previously approved road and sewer projects, and thus result in greatly increased construction costs.

Instead, Detwiler recommended a 17-cent tax cut, a move his Republican elders and Democrats John W. Purdy and Ellen M. Bozman supported.

"I think Steve walks the middle very successfully; he plays it low-key," said school board Chairman Ann C. Broder, a prominent Democrat.

Detwiler, an Arlington native, son of a conservative former County Board member and vice president of First Federal Savings and Loan, would agree.

"I don't perceive myself as a grandstander," said Detwiler, who acknowledges that he has successfully mediated several squabbles between Grotos and Frankland -- one over who would be board chairman and another concerning who would be the county's delegate to the regional Metro board.

"I would very much have liked politically to go along with (Frankland) on the tax cut, but from a more practical business standpoint it didn't seem prudent," Detwiler said.

"Maybe I'm just more moderate on some of the issues than those on either side," and Detwiler, who estimates that he devotes 30 hours per week to his positions as board vice chairman and president of the metropolitan Washington Council of Governments.

Detwiler, who lives in the affluent Williamsburg section of North Arlington, is married and has two teen-aged sons. He holds a degree in business administration from George Washington University.

As the newest member, Detwiler seemed to spend much of his first year on a board watching and listening, and saying little at meetings.

In the past few months, however, his political image has begun to jell and Detwiler increasingly has focused on business issues, particularly the need for economic redevelopment of Arlington's aging Wilson Boulevard corridor, which parallels the subway's newly opened Orange Line.

"Steve is our main liaison with the business community," said Frankland, "and I think he's doing an excellent job."