Even before the Grand Old Party fathers in Virginia could gather in Staunton this Saturday for their great election post mortem and agonizing reappraisal, there is new upheaval in their ranks that is of particular significance to Northern Virginians. On Tuesday, the House of Delegates' Republican caucus dumped Fairfax County's Vincent F. Callahan Jr. as house minority leader in favor of Del. Raymond R. Guest Jr. of Front Royal. While most of the perpetrators of the coup are playing down any monumental philosophical implications, it's hardly an occasion for cheers and confetti in this region.

Mr. Callahan has been a dedicated party moderate who at least until now had enjoyed considerable stature among Democrats as well as Republicans. And next month as usual in the General Assembly, Northern Virginia will have important concerns that can use all the legislative clout this region can muster. Granted, Mr. Guest's home base of Front Royal is not exactly Southside; and he doesn't have to look far up the road to sense the transportation, development and school pressures that top the legislative shopping lists of the Northern Virginia delegation. Increasingly his constituents will share the same interests.

Many of the Republicans who voted for Mr. Guest said they felt a need for more "forceful" leadership and a more active role in the GOP's state party activities. Yet Mr. Callahan's strength was his effectiveness behind the scenes in the legislature -- including a sound working relationship with House Speaker A. L. Philpott. Mr. Guest's chief accomplishment in the legislature is stewardship of a parimutuel horse-race betting bill through the General Assembly nearly eight years ago; the measure was then killed by voters in a referendum.

Now the House Republicans have put their money on a dark horse to pick up the pace. His success may turn on how well he comes to understand the relationship of Northern Virginia's concerns to those of all Virginia.