NOTWITHSTANDING the Sandinistas' quick and defiant rejection of it, President Reagan's new Nicaragua plan marks a step forward in one important respect. It points a way to suspension of the war for at least two months in order to give negotiations a better chance. The regime in Managua had refused to accept the basic terms of the Reagan offer -- a cease-fire and talks with the resistance -- when the resistance put these forward on March 1. But now the official American weight is behind them and, with Colombia's president pronouncing the new plan "positive," Mr. Reagan has some basis for asserting that "we'll have the support of the Contadoras," the Latin democracies that have been trying to mediate.

Not even the president disputes that it was congressional opposition to funding the Nicaraguan contras that led him to his new plan. It reflects his still intense and lopsided focus on keeping them in the field. He insists the Sandinistas won't negotiate unless Congress releases $14 million for the contras' nonmilitary needs right away and leaves him free to fill their military needs if talks don't pan out soon. For this he is being widely accused, by the Sandinistas and others, of demanding unreasonably that Managua negotiate with a gun at its head. This is so, though given the Sandinistas' treatment of their opposition so far, it is not far-fetched for the president to fear getting trapped in a situation where talks drag on and on.

Still, the more relevant question remains what will best move the Sandinistas toward respect for their neighbors and -- much the more difficult and controversial goal -- reconciliation with their fellow Nicaraguans. The Nicaraguan resistance has some notably democratic elements; a spokesman makes their case on the opposite page today. But the three-year record of this American-backed insurgency does not build confidence in its utility as a bargaining lever. It has given the Sandinistas the high ground of Nicaraguan nationalism, undercut mediation by the Latin democracies and kept Mr. Reagan fighting a costly uphill battle at home.

It needs to be underlined that the reluctance of many Americans to support the contras militarily comes not from favor for the Sandinistas but from an objection to reliance on military intervention in Central America. That the Sandinistas also object to intervention does not make it more palatable. The element that the Contadora group is now encouraging in American policy, moreover, is not the threat of a return to intervention in 60 days -- this runs fundamentally counter to the Contadora charter -- but the possibility of a wider ambit for the group's own diplomacy.

It would be a waste if all that Mr. Reagan's new plan produced were a hotter argument over the contras. Believers and skeptics alike ought to try to profit from the fact that he does offer a new way to put together a military policy and the pursuit of a political settlement. He has reshuffled some of the cards. Perhaps some more of them can be reshuffled at the same time.

The prime requirement is an intense common effort to get a prompt and unconditional cease-fire. The initiating side, and its patrons, will deserve much credit. The side that drags will lose accordingly. A cease-fire can save lives, lower the temperature and improve the atmosphere for talks.

One can imagine a clutter of talks and of bids for talks: between Nicaraguans, between the United States and Nicaragua, and in the Contadora group. The Sandinistas and their Nicaraguan opponents have much to talk over, and much to compromise. The United States and Nicaragua could usefully resume the bilateral talks the administration earlier broke off. On the Contadora group, however, falls a special responsibility to use the moment well. Its urgent task is to address the objections that other Central American nations have to the Contadora draft that Nicaragua accepted last Sept. 7. These objections relate mostly to inspection and verification of crucial nonintervention measures bindly on both sides. This seems to us the likeliest place for early progress to demonstrate what desperately needs demonstrating -- that there is an alternative to war.