The Security Council is expected to vote unanimously on Thursday for a resolution urging a "frank and constructive dialogue" among Central American nations, and endorsing the regional peace initiative by Colombia, Mexico, Panama and Venezuela.

An American vote for the mildly worded resolution, introduced today by Nicaragua and seven other Third World countries, was described as "a lead-pipe cinch" by one U.S. official.

He maintained that the 10-day-long backstage negotiating process, which modified Nicaragua's original harsh and condemnatory text, was "a sharp diplomatic defeat" for the Sandinista government.

"They discovered that Third World radicals can no longer automatically drive a wedge between moderates and the United States," the American official said.

But one neutral Latin American diplomat suggested that it was Nicaragua that chose "to dilute the resolution to the consistency of water," to avert an American veto and "put the Security Council on record" as taking a decision on Nicaragua's case.

Two earlier council debates convened by Nicaragua--one a year ago and another in March--ended without the adoption of any resolution. Nicaragua has accused the United States of supporting guerrillas fighting to overthrow the Sandinista government. The Reagan administration maintains that the problem is an internal one of "Nicaraguans fighting Nicaraguans."

"This time Nicaragua sets the precedent of U.N. involvement in Central America, which Washington wanted to avoid," the Latin American diplomat said. "This makes it easier to use the Security Council for the next round."

From the American viewpoint, however, the present resolution avoids injecting the United Nations into the Central American dispute, and furthers the multilateral regional dialogue favored by the United States, Honduras and El Salvador, rather than the bilateral negotiating process sought by Nicaragua.

The original text circulated by Nicaraguan Foreign Minister Miguel D'Escoto would have demanded "an immediate end to the aggressions and interventions, overt and covert, against Nicaragua . . . on the part of a great power."

It would have called for bilateral dialogue between Honduras and Nicaragua and between the United States and Nicaragua.

And it would have called on U.N. Secretary General Javier Perez de Cuellar to use his good offices to help bring peace to the region, in coordination with the "Contadora Group" of Colombia, Mexico, Panama and Venezuela.

The final text eliminates all reference to attacks on Nicaragua, referring instead to both the "situation" along the Nicaragua-Honduras border and "the existing crisis situation in Central America."

It affirms the right of Nicaragua "and all other countries in the area" to be free of outside interference.

At Washington's insistence, it deletes the call for U.N. involvement through the secretary general, simply calling on Perez de Cuellar to keep the council informed of developments.

Tonight the Contadora Group, named after the Panamanian island where the four nations first met to discuss the Central American crisis, were seeking one further minor change in the text.

There was also a discernible change in rhetorical tone among the 35 delegations that spoke in the six council sessions held over the last 10 days. Two months ago, most speakers--including a number of American allies--diverged from the U.S. stand that Nicaragua faced no outside intervention.

This time, despite President Reagan's public statement of support for "freedom fighters" inside Nicaragua, fewer speakers made value judgments. Instead, most limited themselves to calls for support of the Contadora peace effort.