Arlington Democrats last night nominated Mary Margaret Whipple, a former school board chairman, as their candidate in next November's County Board election, thereby expanding their party's role in local politics.

The 124-to-114 vote to nominate rather than endorse a candidate, taken at a mass meeting after an hour-long debate, marked the second time in recent years that the Democrats have taken over a key role in local politics from the nonpartisan Arlingtonians for a Better County.

Last night's victory of Democratic party regulars over the ABC is further proof to some that the reform group has lost its once-powerful grip on Arlington politics.

"It's a big mistake," said Del. Mary Marshall, a Democrat who last night spoke out in favor of keeping partisan politics out of County Board elections. "If it continues, ABC would become just a little club."

The debate between the party regulars and the ABC-ers--most of whom are also Democrats--centers on Arlington's many federal employes, who are barred by the Hatch Act from participating in partisan politics. The ABC has traditionally provided federal workers a vehicle for participation in local government, leaving Democrats merely to endorse their choices for local office.

By nominating rather than endorsing Whipple, the Democrats would be denying their candidate the help of an estimated 150 federal workers, said ABC Chairman Tom Hall last night. "Do not compound the inequities of the Hatch Act by nominating," said Hall, who works at the Labor Department.

On the other side, Democrats argued that Whipple, running as a Democrat, could take advantage of the party's election-day apparatus and, this year, of the Democrats' perceived advantage in a county hit hard by Reagan economic policies.

Most important of all, said several speakers, is regaining control of the five-member County Board, which has been held by Republican-backed members since 1978. The board's majority will be at stake again this fall when County Board chairman Stephen Detwiler is up for reelection. Detwiler, an independent, is expected to receive Republican endorsement.

"If our candidate goes into the race with a split personality--sometimes a Democrat, sometimes not--her chances are diminished," said State Del. Warren Stambaugh (D-Arlington). "What's really important to Democrats is to win this seat."

Whipple, 42, Detwiler's only declared opponent, said after last night's vote that the party will have to make up for the loss of its "Hatched" workers by recruiting "more vigorously than usual" and by putting federal employes to work in such nonpartisan activities as voter registration drives.

Two years ago, County Board member John Milliken became the first board candidate in 25 years to run as a Democrat, prompting some disappointed ABC members to start writing the group's obituary. This year, the organization tried harder to organize against a partisan nomination, sending out 1,800 postcards to members and supporters urging them to come to the Democratic mass meeting and vote for a Democratic endorsement, rather than nomination.