At midterm, the convention process for nominating U.S. Senate candidates in Virginia appears to deserve at least a passing grade.

It is the first time in modern state history that both major parties have resorted to conventions instead of primary elections to choose their nominees for a regular statewide election.

Until about 10 years ago, the Republicans were such a tiny minority of the electorate that it would have been embarrassing to invite voters at large to participate in a GOP primary.

Until this year, the Democrats dismissed the Republican convention as a "closed system." They also turned to a convention this year, however, in recognition of the fact that primaries have become too costly, too divisive and increasingly ineffective in turning out large numbers of election weary voters.

On Saturday, the Democrats will hold mass meetings in every city and county in Virginia to choose delegates to the June 9-10 convention in Williamsburg. It appears likely that none of the eight Democratic candidates will win a majority delegates in this first round and that a substantial number will go to the convention truly undecided.

What this means is that the Virginia Democratic Party is still very seriously examining itself, trying to decide how it can recapture the allegiance of the state's conservative electorate without recanting its break with the arch conservative era dominated by the followers of the late governor and senator, Harry F. Byrd Sr.

The Democrats hope to turn out as many as 25,000 people for their mass meetings. If they do, they will have demonstrated that the party retains a vitality that needs only an attractive candidate to put it back into competition with the Republicans in statewide races.

The Republicans have been holding a few city and county meetings each week since mid-February and will not finish their process until May 16. With about half the delegates to the June 2-3 convention in Richmond selected, GOP leaders are not only pleased with the participation in their meetings, they are slightly amazed.

About 2,000 Republicans took part in Fairfax County and Alexandria, 300 in Richmond and 300 in Norfolk. About 100 turned out in Washington County in rural southwest Virginia, and in Rappahannock County, one of the least populous in the state, 50 Republicans showed up to choose a grand total of three delegates.

Republican city and county leaders are attributing the heavy participation to more than the vigorous campaigns of four senate candidates. They say it is also the result of a general upsurge of interest in the GOP, growing opposition to President Carter in the state and rising number of Republican candidates for legislative and local government offices who draw their followers into the party process.

Only two aspects of the convention process have raised serious objections so far: the Republicans' instructed delegations and on the Democratic side, proportional representation.

Republican candidate Richard D. Obenshain, a former state chairman, succeeded in getting all of the 105 delegate votes in the Richmond suburb of Chesterfield County instructed to support him and is making the same effort in another Richmond suburb, Henrico County.

A delegation can be instructed if a majority of those attending a mass meeting vote to require all delegates to cast their ballots for one candidate even though some of the delegates may actually prefer someone else. Party rules permit instruction, but the Obenshain successes in his Richmond political base as causing some hard feelings with supporters of former Gov. Linwood Holton, former Navy Secretary John Warner and state Sen. Nathan H. Miller.

The Democrats' proportional representation rule requires candidates to win 20 percent of the votes at a mass meeting to get any delegates and then awards them delegates in proportion to the votes they receive.

Long shot candidates such as Falls Church feminist Flora Crater oppose the 20 percent rule on grounds that it freezes them out of the convention. Party chairman Joe Fitzpatrick opposes proportional representation because he thinks it makes the selection process too complex. He wants delegates chosen by a simple majority vote.

These, however, are minor controversies in a process that on the whole appears to be working so well tht it may be a long time before either party in Virginia again resorts to a primary to choose its statewide candidates.