YOU DON'T have to be an admirer of the Reagan administration's Central America policy to wonder what Jim Wright is doing in the Nicaragua negotiations. During Daniel Ortega's visit to Washington last week, the speaker made an intervention into the day-to-day running of foreign policy that was breathtaking in its scope and whose like is hard to recall. Mr. Wright took the play away from the administration on a key issue, and does not seem even to have informed the White House or State Department of what he was doing.

It is true that back in August, Mr. Reagan invited Mr. Wright into a Central America partnership; it was overtaken within days by the Arias plan. Then Mr. Reagan created a huge political opening for Daniel Ortega by refusing to receive him while he was in Washington; Mr. Ortega saw the opening and sped up to Capitol Hill. Mr. Reagan, who meets with Mr. Ortega's opposition and arms it, finances it and keeps it alive, could hardly have been surprised at the Sandinista leader's reach for the comforts and favors of the American opposition.

However, we are not talking about Daniel Ortega but about Jim Wright, who knows how the American system should operate. Political circumstances put the contra-aid card in his hands; that is the basis of the special power he wields in this situation. But the proprieties of the American system come under heavy assault when the speaker uses such power as though the actual conduct of diplomacy in this delicate passage were his responsibility. By inserting himself into a negotiation in a way that keeps the president out, he overreaches recklessly.

Is he also lending himself to a charade at which Daniel Ortega may be more clever than he is? Certainly it would be a disgrace if he were simply helping Mr. Ortega arrange a prompt cease-fire on terms that allow the contras and the democratic elements of their constituency as little political space as possible in the new Nicaragua. For an American opposition leader to end up narrowing the field for the Nicaraguan opposition would be a bitter outcome. Our misgivings, however, do not center now on the substance but on the procedure. Mr. Wright appears to have gone way over the line that separates opposition from interference.