THE PEACE PLAN launched by the American government last week had a stunning immediate result. No sooner was it announced than the five Central American states suddenly agreed, for the first time, on their own plan. They accepted a proposal by Costa Rican President Oscar Arias that the Sandinistas had previously found too helpful to their political opposition and that the United States, El Salvador, Honduras and the contras had found too favorable to Nicaragua -- too favorable in that the Arias plan counts on Latin political pressure rather than contra military pressure to hold the Marxists in Managua to their promises of democratization. Mr. Arias believes that, with the contras shelved, the Sandinistas will have no excuse for further default.

Whether he was wise to take this risk will become evident soon. What is already clear, however, is that the United States is becoming a party to the risk-taking. This comes about through the strange turn that led the White House to enlist House Speaker Jim Wright to build bipartisan support for American policy. By the pact, President Reagan hoped to ensure that if negotiation on the American plan failed, Congress would vote to renew contra aid. But when the Central America five surprised everyone by embracing the Arias plan, the American initiative abruptly lost all prospect of a Latin interlocutor. Administration officials alternately grumbled and put on a good face, but their new congressional partner charged straight ahead. The Arias plan ''has to prevail,'' Democrat Wright says; the American plan is ''merely supportive.''

In fact, Mr. Wright has become something of an extraordinary player in the Central American events. Not only does he anoint the Arias plan. It turns out he personally negotiated, with a contra leader, the sensitive question of who is to speak for the contras in cease-fire talks: the Catholic Church or the Red Cross. From Nicaragua's vice president he secured a pledge of free elections in return for an end to contra aid. From Nicaragua's ambassador he received assurances on getting rid of the Soviet and Cuban military presence.

Mr. Wright, who now commands the swing vote on contra aid, insists he has no explicit or implicit agreement with the president on this issue. He wants to hold contra aid off the agenda for a few months to remove what he regards as the pall of threat from negotiations. He's taking a large chance, but a necessary one, in helping open the way to convert the Arias plan from paper to reality. The danger is not that Ronald Reagan is making a fake pass at negotiations, as some Democrats were quick to assert, but rather that the Sandinistas will cheat. It is in bringing pressure on them on this score that Mr. Wright's contribution needs to be made