Virginia's two Republican candidates for governor clashed today over the abortion issue and the exchange was so sharp that it appeared to have scuttled the party's plans to have the candidates sign a unity pact.

" . . . You are treading alarmingly close to the boundaries of honesty and truth," charged the chairmen of Rep. Stan Parris' campaign in a letter to candidate Wyatt B. Durrette. "This attempt to mislead the people . . . causes us to have a great deal of concern about the type of campaign you are conducting . . . . " said John Alderson and William Stanhagen in the letter.

" . . . It is not only the facts . . . but Stan Parris' credibility that is at issue," shot back Durrette.

The two contenders for the GOP nomination are fighting over who is most opposed to abortion. Dick Leggitt, one of Parris' closest aides, said today that Parris was so upset that he would not attend a planned unity meeting Monday at GOP state headquarters here.

Both candidates were supposed to sign a pledge that each would not unfairly attack the other during the campaign -- the so-called 11th commandment of the Republican Party.

Parris touched off the exchange when his campaign charged that Durrette's literature falsely described a 1970 Parris vote in favor of a new abortion law as a vote to liberalize abortions in the state.

Durrette, a former Fairfax legislator and now a Richmond lawyer, said Parris, then a member of the Virginia House of Delegates, voted to allow abortions in cases of rape, incest or when the mental and physical health of the mother is endangered. That, Durrette said, was more liberal than the previous law, which permitted abortions only to save the life of the mother.

"Stan Parris cannot deny that he voted for a bill that liberalized abortions in Virginia," Durrette said. He noted that the number of abortions in the state rose from 300 a year to 3,070 after the 1970 law became effective. Durrette was not a member of the assembly at the time of the vote.

Parris contended that the 1970 law stopped "abortion on demand" by physicians who declared the operations were emergencies. Parris also said he has run with the support of antiabortion groups, including 1982 when "prolife groups campaigned door to door" for him.

Durrette, in a separate campaign flier released today, said he approved the attack on Parris only after Parris had distorted his positions on other issues in earlier mailing.

"Stan flooded the mail with highly negative attacks on my record," Durrette said. "Such calculated misrepresentation would be unacceptable even against a Democrat," Durrette said, "but against a fellow Republican, it is intolerable."

The party's nominee for governor will be chosen at a state convention May 31-June 1, but delegates to the convention will be selected during a series of local mass meetings begining in early February.

Durrette has been regarded as the party's front-runner, but today's exchange was another indication that Parris would not lose without a spirited fight.

"It sounds like it's heating up," said state GOP Executive Director Sandy Scholte.