The occasion was an intimate luncheon briefing at the White House for a handful of correspondents. The subject was the Reagan administration's new plans for fighting the "brutal and totalitariam colonialism" of international communism in Central America. And the setting was the (Teddy) Roosevelt Room.

What more could you ask? It was in the Caribbean, on Cuba's San Juan Hill, where the Rough Rider made his name. It was on the Central American mainland where he won the hearts of his countrymen by wrenching away a slice of Panama, proclaiming pseudo-sovereignty there for the United States, and building a canal. It was in that same imperial spirit, more than 70 years later, that Ronald Reagan charged about the country on his way to the presidency, crusading in vain against those who would undo Roosevelt's land grab by handing it all back to Panama.

When, to all that, you throw in a cameo appearance in mid-briefing by the California Rough Rider himself ("I don't want to interrupt, but . . ."), the metaphor would seem to be just right.

In fact, it doesn't work. And the reason is in the tone, as well as the content, of the administration's new social and economic emphasis in its approach to Central America. Before the president dropped by, with Vice President George Bush in tow, we had been assured by Tom Enders, the assistant secretary of state for the hemisphere, that "the battle for El Salvador is the essential struggle in Central America; if the leftists win, the jeopardy would be immediate and massive to the rest of the area."

But almost everything else said by Enders and the president's trade representative, Bill Brock, had conveyed a strong sense of the limits on the power of the United States to influence the course of events or dictate a quick or decisive answer. "There is no substitute for a grinding effort," Enders said at one point. The trade-and-aid programs offer no quick fix for El Salvador; "We should have been into this issue a long time ago."

The $350 million in economic aid is a one- shot affair, with almost one-third going to El Salvador and most of the rest to Costa Rica and Jamaica. The tariff relief will apply to only about 13 percent of trade from the region; the rest is already tariff-free. It is designed, together with the investment tax incentives, as a long-range inducement to American business to build or expand enterprises in Central America.

Military aid was scarcely mentioned. Brock said the $60 million figure (more than half for El Salvador) was "consciously kept out" of the president's major address to the Organization of American States a day or so earlier so as not to sour enthusiasm for the social/economic side of the effort.

As for the question of American combat involvement, it didn't come up until the president arrived. How far, he was asked, are you prepared to go? Reagan passed up the invitation to restate the whatever-is-prudent-and-necessary principle. He had said in his speech that "we will not follow Cuba's lead in attempting to resolve human problems by brute force." As to what "we will or will not do" militarily, he believes "the old historic image of the big Colossus of the North, and gunboat diplomacy, is still engrained in many of those people. . . . Not even our friends of today would want to see that kind of an intervention on our part."

Hardly the language of the bold figure, astride a prancing black horse, glaring down upon us from an oil painting on one side of the room. Not that we didn't get into the domino theory--"a phrase proven true in Southeast Asia," Reagan said, and as applicable to Central America.

But he talked at greater length of "correcting the social and economic inequities (so that) certain portions of the populations in many of those countries" would be less susceptible to the "promises (from) outside, the subversion of telling them there's a better life for them if they follow that other philosophy."

There was, in short, in this informal, off-the- cuff presidential presentation a sense of pace and balance, of preventive measures and sensible planning for a long haul. It was reflected by Enders when he was asked whether "we are winning or losing." His reply: "Neither one--the key is staying power."

To some ears that may evoke "quagmire"-- as in Indochina. Others may see only window- dressing for an unpopular, military-oriented, Salvadoran involvement. But it strikes me as a welcome complement to the one-note bugle-call of the administration's initial pronouncements on Central America.

Reagan said the new economic emphasis had its beginning in a meeting at Camp David last June with Mexican President Lopez Portillo.

But the agenda item at hand was not the Soviet threat, or Cuba's surrogate role with Nicaragua in the subversion of El Salvador, or dominoes--on none of which the two men see eye-to-eye. It was the swelling, not to say flood tide, of immigration (legal as well as illegal) from Mexico and other Caribbean lands. It had occurred to him, the president told his Mexican counterpart, that "maybe they'd be more content to stay in their own countries if there were more economic opportunities."

The fact that he opened this particular mini- briefing with the "illegal immigration" argument is revealing on two counts. One has to do with a widely shared concern with Hispanic immigration dating back to the administration's earliest days. The second has to do with practical politics.

First, the politics: Rep. Michael Barnes (D- Md.), the chairman of the House subcommittee on inter-American affairs, says he was only half-joking when he recently suggested to the president that the new Central American aid program would go over a lot better in Congress if it was called "The Anti-Illegal Immigration Act of 1982." His point was that the program's inevitable entanglement with current policy in El Salvador makes it vulnerable to the increasingly potent "Vietnam analogy."

But the "illegal alien" connection, Barnes believes, works powerfully in favor of a heavy effort to rescue the shattered economies of most Central American nations. "There are 110 million people living between Panama and the Rio Grande, and the number is likely to double by the end of this century," says Barnes. "We are going to have an explosion of illegal immigration in this country."

Such is the congressional concern, Barnes contends, that the president could get more money "from my side of the aisle" than he has asked for. And Barnes would rest his case on home-grown forces driving Central Americans to seek haven in the United States: higher oil prices, economic mismanagement, slumping prices for coffee, cotton, copper, sugar and other produce which are often a sole means of support, soaring unemployment, illiteracy, grinding poverty, violence and/or repression from the left (Cuba, Nicaragua) and the right (El Salvador, Guatemala).

Secretary of State Alexander Haig comes to an even more apocalyptic conclusion than Barnes by taking the East-West ideological route. In a recent speech to the nation's governors, many of whom express alarm over the problem of illegal immigrants as it now exists, he gave a foretaste of what will likely be a big administration talking point in the months ahead.

Noting that in 1980 alone, 1.5 million "undocumented" immigrants came to the United States (125,000 from Cuba), he warned: "Just think what the level might be if the radicalization of this hemisphere continues with the only alternative a totalitarian (Marxist-Leninist) model in one state after another. Why, it would make the Cuban influx look like child's play."

You don't, in short, need Fidel Castro to make a case for the Reagan administration's all- too-modest new effort to deal with the problems of Central American immigrants by going to its source.