The last hurdle between Mary Margaret Whipple and her reelection this fall to a second four-year term on the Arlington County Board is expected to come this Tuesday when she will seek the endorsement of the Arlingtonians for a Better County.

Whipple, a popular Democrat who won her party's nomination for the board seat earlier this week, will not have a Republican opponent in the Nov. 4 election.

But she will face competition from unlikely quarters -- the Tuesday convention of the ABC, a nonpartisan political organization that is a kindred spirit to the local Democratic Party. Tom Hall, a one-time chairman of the ABC who has been active in the organization for 17 years, has filed against her.

Although billed as a nonpartisan political organization, the 850-member ABC nevertheless has a long history of joining with local Democrats to endorse County Board candidates who run as independents. But in recent years, board candidates have increasingly opted to run as Democratic nominees instead of independents.

The candidates have still won ABC's endorsement, but their decisions have effectively eliminated any active participation by a large number of ABC members who are federal employes barred by the Hatch Act from taking part in party politics.

Of the four board members who compose the Democratic majority on the five-member board, only Ellen M. Bozman has chosen to run as an independent with the endorsement of the ABC and local Democrats. Board member Michael E. Brunner also ran as an independent in 1983 when he was a federal employe, but with the backing of county Republicans.

Frustrated by the decisions that in recent years have let ABC members participate in local politics only once every four years, Hall said he would challenge Whipple this year on principle.

"I understand Tom's position . . . his frustration," said Paul Michl, ABC chairman. " But Mary Margaret has a lot of support in the ABC . She's a popular incumbent."

Although he concedes there is little likelihood of winning ABC's endorsement, Hall said the challenge is important so federally employed ABC members don't "lose the opportunity and responsibility to excercise the few political rights left to them under the terms of the Hatch Act."

Whipple also is unhappy with the restrictions the act places on federal employes. "Obviously, I wish we didn't have it for the strictly local campaigns," she said, adding that its primary purpose "rightfully" is to "keep pressure off employes" by candidates for national office.

Although she left the nomination-or-endorsement decision to the Democratic and ABC conventions in 1982, Whipple said she prefers to run as a Democrat and has become increasingly active in party politics for national, state and local candidates.

She said that federally employed ABC members could still give indirect support to her campaign through the use of bumper stickers and contributions. Many ABC members, she added, are retired federal workers who can actively work in her campaign, "although there really isn't go to be a campaign to be active in this year because of the Republicans' decision."

Whipple said that Hall could have made his protest in other ways. "What he's really doing is challenging me for the endorsement" instead of challenging the Hatch Act, she said.

The convention begins Tuesday at 8 p.m. in the Little Theater of Washington-Lee High School.