THE ARGUMENT over Speaker Jim Wright's role in the Nicaraguan talks was taken to an unhappy new place over the weekend by Jimmy Carter. The former president met plenty of frustration at the hands of Congress in making his foreign policy. Yet he now states a position that broadly justifies congressional intervention in a diplomatic negotiation, justifies it on terms that promise only grief to presidents. ''Had the president and secretary of state been carrying on their duties to try to enhance the peace in Central America rather than being the major obstruction of peace, then I don't think the speaker would have to take the actions he did,'' Mr. Carter said. Translation: Mr. Wright perhaps behaved questionably, but the president made him do it. Can you imagine any Congress that could not find enough fault in something a president did to justify the act of its choice?

But the Carter contribution to the unending and unendable argument over the powers of the president and Congress was not the most interesting aspect of his remarks. Jimmy Carter is not your ordinary critic of Nicaragua policy. He is a former president and the one on whose watch the Sandinistas took power. His dream was to see Nicaragua make a peaceful transit from dictatorship to democracy, a former aide, Robert Pastor, writes; his nightmare was the Sandinista military victory that arrived. As he left the White House, he felt compelled to resume aid to the heretofore proscribed military regime in El Salvador, victim of a Sandinista-sponsored attack he had desperately hoped would not come.

At that pre-Reagan moment, by a finding Mr. Carter could not have found it easy to make, the Sandinistas were the spoilers. But now Mr. Reagan is ''the major obstruction of peace'': he is hanging too tough with the contras. But let us all confess a little here. We have thought and said that support of the contras was the wrong policy from the start. But it takes a willful blindness not to grant that the contras, besides the bad they did, contributed to the pressure that led Managua to sign the peace plan. The contras appear to be a wasting military asset -- to Mr. Reagan's anxiety and regret. That leaves as the (relatively narrow) policy question how best to phase the contras into a political role as aid runs down.

Mr. Carter had his own experience in trying to steer armed Nicaraguans toward democracy. He should know the perils of the process better than most. When he left office, the sky was darkening in Central America. Now, after a very long night, it may be lightening. For that, Ronald Reagan is ''the major obstruction of peace''?