For all practical purposes, the presidential nominees of both major parties have already been chosen, making the remaining primaries more pro forma events than real contests. That has been the case for the Republicans since Super Tuesday March 8 and for the Democrats since New York's primary April 19.

This means a lot of voters will have been dealt out of the decision-making process by the time the primary season ends on June 7. There has to be a better way.Actually, the Democrats tried to invent one this year by encouraging development of Super Tuesday, one primary date for most Southern and border states. The Republicans followed suit, establishing a one-shot regional primary.

But this merely increased the distortion that early contests usually work on the later ones. The losers of the early primaries in small, unrepresentative states have historically been unable to continue. When faced with a 20-state race on one day, doing so became all but impossible.

It arguably would have been better not to have invented Super Tuesday at all. That way, the losers might have had a better chance to catch up in the next round. But not a good chance, as history shows. Very few candidates in modern political history have won their party's presidential nomination without winning the New Hampshire primary. So abolishing Super Tuesday isn't the answer.

But one fact is clear: single-state and regional primaries don't mix; they make things worse.

It is tus essential to try something else. Democratic Sen. Alan Dixon of Illinois has a plan worth considering: regional primaries for the whole pre-convention system.

His bill, introduced last fall, would establish six regional primaries, one to be held every other week from the last Tuesday in March to the first Tuesday in June. The order in which they occur would be chosen by lot three months in advance of the first, preventing candidates from concentrating on only one geographic area of the country for several years before the election.

The regional primary concept has flaws, of course. It doesn't solve the problem of money. The best-financed candidates usually win the early contests, as they did this year, making it hard for the rest to go on raising money to have a chance to win later. To be sure, money didn't entirely determine the Iowa and New Hampshire results in 1988, but it was certainly decisive on Super Tuesday and thereafter.

A regional primary system would still give the advantage to candidates who could raise the most money. Victory in the first couple of regions to vote might well establish the momentum to win the rest.

But without small, unrepresentative contests before the first regional contest, the less-well-heeled contenders would have a fairer shot in the first truly meaningful arena than under the present system, or under the old single primary arrangement. That is reason enough to give regional primaries a try in 1992.