Up to now, Virginia's contest for a U.S. Senate seat has been full of suspense, with political high dives, back flips, somersaults and cannonballs in and out of the action. But with the reduction of the contest this week to a mere two men--Democrat Richard J. Davis and Republican Paul S. Trible Jr.--the real fun may have just begun.

For the first time in a half-century, Virginia will not have a Harry Byrd to send to Washington--and that in itself will dramatically change the way of doing political things in the state. Mr. Byrd adhered to his original decision to retire. In doing so, he left the field to two candidates both unlike himself and unlike each other:

Mr. Trible, while sharing more than a few views with Sen. Byrd, comes from a different political school. At 35, he is a down-the-line party member who has risen quickly through the ranks of a newly established statewide Republicanism. Mr. Davis, at 60, enjoys certain support that the old guard in Virginia has never cared much to court, as well as backing from party regulars who refused over the years to bolt for any of the "independent" movements.

Mr. Davis has moved quickly, too, to make President Reagan's economic policies the central theme of his campaign. Mr. Trible, on the other hand, has been a loyal supporter of the Reagan administration. So even though the voters' attitudes toward local and state issues tend to have as much influence as anything else on these contests, not to mention their response to the personal qualities of the candidates, the Senate race in Virginia could become a crude measure of how the people of this state view their world under President Reagan.

That, in turn, should be enough incentive for each national party to sit up, take notice and send money. That, plus the fact that this does not look to be one of those me-too affairs in Virginia politics, could make the next five months as engaging as the sideshows that led up to it.