Key Biscayne, Fla. — To the obvious delight of the Reagan administration, more and more Latin American countries are identifying themselves with Washington's warning that Nicaragua poses a major threat to hemisphere security.
But almost all the concern about the situation in Nicaragua is accompanied by warnings against unilateral United States moves against the Central American country.
That is the same message that Venezuelan President Luis Herrera Campins gave President Reagan two weeks ago when he visited Washington.
The Venezuelan leader expressed his growing concern about ''the drift toward tyranny in Nicaragua,'' as it was phrased by a Venezuelan spokesman.
But Mr. Herrera Campins insisted that military intervention would damage hemisphere relations. And that is Mexico's view and that of many of the Caribbean leaders assembled here for the fifth annual Miami Conference on the Caribbean.
So what Washington has is a half a loaf - Latin America, more and more siding with the US on the dangers inherent in Nicaragua today, but solidly against any precipitous action against those dangers.
At the same time, the Reagan administration thinks it has gained ground diplomatically on Nicaragua - possibly turning a corner in trying to convince Latin America that the presence of Soviet military hardware in Nicaragua is a major threat to hemisphere security.
This is the line that Secretary of State Alexander M. Haig Jr. has been touting for weeks, but it has not won much support in Latin America - until this past week.
Now the administration feels it is winning that support.
The turning point, say high administration officials, was Mr. Haig's trip to Mexico City last week.
While the Mexicans told Mr. Haig that they would vigorously oppose any unilateral US move against Nicaragua, they also indicated, according to Washington sources, that they share the US concern over recent developments in Nicaragua. Specifically, it is reported that Mexican officials agreed with Mr. Haig that the Sandinista government in Nicaragua is headed along the path toward totalitarianism. They also said they were worried about the military buildup in the Central American country.
These US sources say that Mexico subsequently expressed its concern on these issues to Nicaraguan Foreign Minister Miguel d'Escoto Brockmann when he visited Mexico City this past weekend.
What apparently prompted the Mexican about-face were the jailings of opposition leaders, repeated closings of the newspaper La Prensa, and the presence of growing quantities of Soviet armaments in Nicaragua, including perhaps as many as 100 tanks and possibly MIG jet fighters.
Thus the US position on Nicaragua has won significant, albeit unofficial, support at the conference here. Several island leaders in private conversations said they saw the present situation in Nicaragua as ''intolerable,'' or, as one put it, ''a slow drift toward the sort of tyranny that exists in Cuba.''
Some Caribbean leaders have indicated that Washington would do well to give this growing hemisphere concern about Nicaraguan trends an opportunity to dawn on the Sandinista leaders in Nicaragua.
''They can't be so stupid as to fail to recognize it,'' said an island leader who has visited Sandinistas on several occasions.''Give them some time to think it through, as well as some space to maneuver and change.''