NEXT YEAR about three-quarters of the delegates to the Democratic and Republican national conventions will be chosen in some three dozen presidential primaries. As recently as 1968, only about 40 percent of convention delegates were products of state primaries, of which there were only 17. How did presidential primaries become such a growth industry and what are the implications of their growth?

The source was the 1968 Chicago Democratic Convention. That wretched gathering of Democrats adopted the minority report from the convention's rules committee, requiring state parties to guarantee the selection of future convention delegates "through a process in which all Democrats have full and timely opportunity to participate," and it mandated the creation of a Commission on Delegate Selection.

That commission, after hearings in 17 states, laid down several general rules by which everyone would have to abide. They required that the delegate selection process begin in the calendar year of the election, outlawed slate-making and proxy voting, and insisted that the party meetings be open to all Democrats.

Hardly the stuff of political revolutions. But the commission will be remembered (and also reviled) among Democrats for one further change: it required state parties to take "affirmative steps to encourage . . . representation of minority groups at the national convention in reasonable relation to the group's population in the state." The same requirement was imposed on state parties in regard to people under 30 and to women.

This condition -- that one result of an open process be to reflect a formula balance of blacks, women and young people -- was almost immediately seen as an invitation to challenges of entire delegations at future conventions. Beginning in 1970, many state parties concluded that because national conventions have historically been reluctant to overturn primary election results, they themselves would be willing to accept less influence in the selection of delegates in return for fewer challenges to their delegations. The next step was an increase in the number of presidential primaries: 23 primaries in 1972 grew to 30 in 1976 and grew again to 36 for 1980.

This primary growth has been both unprecedented and unplanned. Electoral sprawl will reach the point of political chaos on June 3. On that date, presidential primaries will be held in eight states: California, Ohio, New Jersey, West Virginia, Rhode Island, Montana, New Mexico and South Dakota. Of the 1,666 delegates needed for nomination at the 1980 Democratic National Convention, 696 will be chosen on that first Tuesday in June.

Is there any more foolish use of a candidate's time (or of aircraft fuel) than the hopscotching by jet from Camden, N.J., to Columbus, Ohio, and on to Cucamonga, Calif.? If the nation did not already face a serious energy shortage, the schedule of the 1980 presidential primaries would provide us with a beaut, all by itself. It does not seem too much to ask of the two major political parties (especially the Democrats) that spawned this explosion of primaries that in the future primaries be held on the same day only in a single time zone. It would save fuel and wear and tear on the candidates, and might even encourage some face-to-face campaigning in the same place, after Iowa -- not to mention the obvious savings in campaign money. But the subject of campaign spending in relation to primary sprawl is the other half of the subject and needs to be taken up on its own.