TIME WAS WHEN nearly all political eyes in Northern Virginia would be trained on every primary and general election for Congress in the Tenth Congressional District--where for more than two decades the name of the game was Joel T. Broyhill against anybody else. A string of Democratic candidates, each banking on the law of averages and his own record to win, went down to defeat and back into private life. Finally, in 1974, Joseph L. Fisher defeated Republican Broyhill. Mr. Fisher went on to win two more elections before losing in 1980 to Republican Frank Wolf. But today, few besides party activists seem aware that the Democratic primary for this seat is set for next Tuesday.

Those politicians who are paying any attention figure that relatively few of the district's 225,000 voters are expected to cast ballots. This could mean that a small number of votes could decide who gets the nomination, since three Democrats are competing for the chance to run against Rep. Wolf. Complicating the Democratic contest still more--or making it less important in minds of some voters, perhaps--is the fact that all three candidates agree on almost everything: they favor a nuclear freeze, oppose the Reagan administration's budget cuts and have spoken against policies that have cost the jobs of federal employees.

The candidate regarded as the "front-runner"--on the basis of experience and name recognition--is former state delegate Ira M. Lechner, a labor lawyer and Arlingtonian. The other two are first-time candidates: Rose Thorman, also from Arlington, who has been women's program manager at the Bureau of Mines, and Edward McLaughlin, a McLean media consultant.

Perhaps all the statewide attention attracted to the Democratic search for a U.S. Senate candidate has simply upstaged the congressional primary. That search is scheduled to end one way or another with a convention decision on Saturday, and maybe then more voters in the Tenth will decide they have an interest in who might wind up representing them in the House for the next two years--or more.