Two Republicans on the Arlington County Board took to the courthouse steps yesterday to accuse Democrat Mary Margaret Whipple of using "false statements" and "non-issues" in her race to unseat their GOP colleague, Board Chairman Stephen H. Detwiler.

"For too long, Mrs. Whipple has been making very strong statements that are unfounded and without any proof of anything," said board member Walter L. Frankland Jr. at the news conference with his colleague Dorothy Grotos. Both attacked Whipple for her charge that the board's Republican majority had resorted to "backroom politics" and questioned whether she has the experience to run for the board.

Detwiler, seeking a second four-year term as an independent with GOP support in a race that will determine the board majority for at least another year, wasn't at the press conference and said later he had nothing to do with it. But the 39-year-old savings and loan executive said he was grateful for his colleagues' help.

"I have felt from the outset that Mrs. Whipple has run a negative campaign, that she has been misleading the citizens and manufacturing her own facts to support her own whims," said Detwiler.

Whipple, 41, a former chairman of the Arlington County School Board making her second bid for a county board seat, said the Republicans are off base. "I feel I have conducted a very issue-oriented campaign," said Whipple. "Yes, I have criticized Mr. Detwiler's record when it should be criticized. It is his word that it is negative."

She also bridled at the Republicans' questions about her qualifications. "I was a chairman of the school board before Detwiler ran for the county board," said Whipple, who until recently was given little chance of unseating Detwiler, considered a moderate on the board. "It seems to me they're sounding nervous."

The renewed hostilities between Republicans and Democrats in Arlington have resurrected a year-old issue: the August 1981 firing of County Manager W. Vernon Ford. The Republicans, who assumed control of the county board in 1978, discharged Ford, prompting an immediate retort by the Democrats that the dismissal was the product of "backroom politics."

That charge has surfaced as the centerpiece of Whipple's claim that the Republican majority has "threatened" the county's tradition of "open government." She has also criticized their handling of school board appointments.

At the press conference yesterday, Frankland and Grotos flatly denied Whipple's account of the Ford firing. "I would like them to show proof where there has been closed-door meetings," said Grotos.

Whipple said yesterday she rests her charges on a private meeting between Detwiler and Ford at which, she said, Ford was "told to resign or be fired." "On what basis, he Detwiler said that, I don't know," said Whipple. Detwiler has confirmed the meeting with Ford but yesterday denied he gave the manager an ultimatum to resign or be fired.

"There was no inappropriate action," said Detwiler. "We were totally open and followed the proper procedures from start to finish." Detwiler would not say more on the firing, noting that Ford has filed a lawsuit for damages over his discharge.

Detwiler, whose 1978 election gave the board its first Republican majority, has campaigned on the lowering of county taxes since the Republicans took control with his first election in 1978. Since then, the real estate tax rate per $100 valuation has dropped from $1.49 to 98 cents; it rose from a 96-cent low last year with Detwiler casting the key vote for the 2-cent increase.