Mary Margaret Whipple and Charles Rinker won endorsement last night of the Arlington Democratic Party as independent candidates for the Arlington County Board.

Whipple and Rinker will face incumbents Walter Frankland and Dorothy Grotos, who are expected to receive Republican endorsement. The November election will determine whether control of the five-member Board remains in the hands of Republican backed members.

At a mass meeting attended by 420 persons at Wakefield High School, Whipple, chairman of the Arlington School Board, and Rinker, an employe of the federal ACTION agency and president of the nonprofit Arlington Housing Corp., challenged what they called the current "mistake-a-month" board majority which has been in power since January.

Accusing the GOP-backed majority of "jeopardizing good government," they said, "they carp and complain and criticize, but they do not offer constructive solutions to our problems."

Whipple and Rinker won the Democratic endorsement over William Lightsey and Julius Brenner and are also expected to receive the endorsement of the nonpartisan Arlingtonians for a Better County (ABC) organization at a meeting Monday.

Rinker accused the Republicans of "turning a deaf ear to the critical needs of moderate-income tenants . . . the unemployed and economically disadvantaged to minorities, to the elderly . . . small businessmen and women."

Whipple said the greatest task the Democrat-backed independents face is to "make the people understand that the recent tax cut the board granted came only because of us. We already had one of the lowest tax rates in the area before the Republicans came to power and we left them a surplus to cut it further."