President Reagan, looking bright and sporty in an oyster-white linen blazer, navy trousers and bold striped tie, stopped by a gala "Caribbean Evening" last night to lend support to his Caribbean Basin Initiative, which has been having some trouble in Congress.

At least 1,200 diplomats, politicians and business leaders crowded into the Organization of American States Building to drink pin a coladas and finish off a day devoted to heavy lobbying for Reagan's $350 million plan to bring economic relief to the Caribbean nations.

In a brief speech, Reagan said the Caribbean Basin Initiative is needed to "face the grave economic problems and the political instability and, sometimes, subversion that feed on these problems.

"With the Caribbean Basin Initiative, we can open a window into a different time, a different future," Reagan said, adding that a recent discussion with political leaders from the area had indicated the "economic crisis in the area had worsened."

Reagan called the initiative a priority and promised to push for approval before Congress recesses for summer.

The party itself was a zoo. Moving from one end of the vast room to the other took some serious planning, shoving and sweating. A rather loud Jamaican steel band forced guests to scream their greetings to each other. Because of the band and the screaming, organizers decided to dispense with the planned program after Reagan spoke.

Tourist displays lined the walls, offering everything from Antigua rum to Jamaican cigars to fancy week-long vacations in St. Lucia for $378. Brochures abounded.

Sponsored by the CBI Coalition, a newly formed lobbying effort aimed at gathering public support for the president's program, the party raised $60,000 to help the group in its efforts. Individual tickets sold for $100.

The food tables, surrounded about 10-deep by hands grabbing and toothpicks poking from all angles, contained native foods such as empanadas, banana bread with macadamia nuts and tropical fruit platters. Most of the food came from Ridgewell's, with a few select native dishes from some embassies.

"There's about four dishes out there that were not appropriate for cocktail party--spicy beef and chicken, a bean dish and a rice dish," said a woman from Ridgewell's. "So I stuck them out there with some crackers. What else could I do?"

Earlier in the day, delegations from 18 Caribbean countries descended on the Hill, each assigned two members of Congress to lobby. While the House Foreign Affairs Committee gave the administration a partial victory by approving a version of the plan last week, the measure faces uncertainty on the House floor, where all types of foreign aid are unpopular during an election year.

"It's a very important part of U.S. foreign policy, and we get all the benefits from having strong and healthy foreign neighbors," said Secretary of Defense Caspar Weinberger. "It's important because they're so close to home. They're in our back yard."

"It's a must for the U.S. to support this," said OAS Secretary General Alejandro Orfila. "You get solidarity, you get security, you get raw materials--and it makes for stronger Americas all around."