Arlington Republican leaders announced last night that the party will not field a candidate in this fall's County Board election, apparently clearing the way for Democratic Board Chairman Mary Margaret Whipple to run uncontested.

Helen Blackwell, chairman of the Arlington Republican Committee, said it is "conceivable" but unlikely that someone could run as an independent and seek the party's endorsement instead of nomination.

GOP committee member Thomas W. Dennison, an outspoken critic of the local committee, ruled himself out of a race against Whipple yesterday, the filing deadline.

Dennison had said he would file for the Republican nomination in the County Board race if the party failed to find any other candidates. But he said yesterday that a campaign "would not be worth the effort" because party stalwarts are more interested in helping Rep. Frank R. Wolf (R-Va.) win reelection.

However, Dennison, a self-described moderate, is seeking the committee chairmanship, which Blackwell is vacating. His opponent is heavily favored Scott McGeary, a conservative and longtime party activist. The chairman will be selected at the party's May 10 canvass.

"I'm a little disappointed," Whipple said of the prospect of having no opponent this fall. "I think it's healthier to have a contested election and debate the issues."

Whipple, who is seeking a second four-year term on the board and is serving in the rotating board chairmanship, will face a challenge at the May mass meeting of the Arlingtonians for a Better County (ABC), a political group that endorses County Board candidates.

Tom Hall, a former ABC chairman, said he will oppose Whipple for the group's support in an effort to open the selection process for ABC's many federal employes who cannot work for party candidates.