THERE HAVE BEEN far too many changes in our political rules in recent years, but I'd like to propose just one more. This latest change is so sweeping that perhaps it can erase some of the damage of the others.

It's a simple proposal, really: For the remainder of this year, the only citizens permitted to vote in Republican primary elections shall be Democrats. And, of course, vice versa. Republicans can vote only in Democratic primaries.

Before you object, just think about it. The point of reform, we are told, is to adjust our rules to changing reality. And what is the manifest political reality today? That America is a nation of Democrats, most of whom are anxious to vote for a Republican this fall.

That being the case, it seems only fair to let them choose the candidate of the party that they're going to support.

Republicans should not object to this plan. It might well be the salvation of the GOP. When left to choose their own candidates, Republicans often blow it. They tend to go for someone of such ideological purity that he can't win. The Democrats are generally more pragmatic in this regard -- at least in years when victory is possible.

Republicans, moreover, should be very pleased at being allowed to choose the Democratic nominee. Most Republicans I know are motivated by one or another of two great passions this election year: the election of a true conservative, and the defeat of Edward Kennedy. They could satisfy both these lusts by supporting Jimmy Carter in the Democratic primaries.

Reformers always argue that their schemes should be advanced regardless of the specific results. Nevertheless, we may speculate on what the results might be were Democrats to choose the Republican nominee and Republicans the Democrat.

It is likely that the candidates of the two great parties would be George Bush and Jimmy Carter. If the Republican convention were composed of Democrats, Bush would win in a walk. It is true that his political philosophy is very close indeed to that of Ronald Reagan, but liberal Democrats have long since replaced concern for issues with a moth-like attraction to image -- an Ivy League background being his keenest possible flame.

Either Bush or Carter would be a far more effective president if chosen by the members of the opposition party. The national consensus so long beyond our leader's grasp would finally be attained.

Of course, this particular choice of standard bearers is probably just what we could get if we failed to pass the new reform and let each party choose its own candidate.

Which proves that this proposal is solidly in the tradition of all the other political reforms of the last decade: It wreaks havoc with the process, but leaves our problems exactly the same.