For President Reagan last night, the world in microcosm was right in the White House, where he could reach out and touch almost every trouble spot around the globe.

It was the Reagans' annual party for chiefs of diplomatic missions, and with 268 foreign guests and 50 American officials jammed into the East Room, touching trouble spots was no problem at all.

Take, for instance, Nicaragua, represented by the charge' d'affaires and the permanent representative to the Organization of American States. Earlier in the day, the House intelligence committee voted to cut off covert aid to Nicaragua.

"I hear we got a setback from the House committee," Reagan told Richard Stone, the former Florida senator Reagan has nominated as his special envoy to Central America. When reporters asked what he thought of the committee's action, Reagan's jaw tightened.

"We'll keep right on fighting," he said. "If they committee members want to be irresponsible, that's their business. What we're doing is perfectly proper."

Then there was Lebanon. Reagan told Lebanese Ambassador Khalil Itani he had high hopes that Secretary of State George P. Shultz would successfully wind up his Mideast negotiations on an agreement with the Israelis for a troop withdrawl.

"Tomorrow will be a crucial day," Itani said. "The meeting in Jerusalem will be a decisive one."

Itani said he asked Reagan if Shultz would go to Europe next week for a previously scheduled conference, and the president said he wants Shultz to reach an agreement in Lebanon.

Holding up crossed fingers the way he said Reagan did, Itani said the president told him, "Let's keep our fingers crossed" that an agreement might come by the end of this week or early next.

"I felt the president is sincerely determined to settle the Lebanese problem," said Itani.

Soviet Ambassador Anatoliy Dobrynin said he had no time to discuss substantive matters with Reagan as he went through the Blue Room receiving line. But talking later to reporters about Soviet leader Yuri Andropov's proposal yesterday to reduce the number of nuclear warheads in Europe, Dobrynin said, "Of course it's new."

"Before, we counted planes and missiles. Now it's warheads. I don't know what else to count," said Dobrynin, expressing hope that an agreement might be reached on the superpower issue by the end of the year.

He also said his office extended an invitation yesterday to Samantha Smith, 10, of Manchester, Maine, and her parents, Arthur and Jane Smith, to make a trip to Moscow this summer, paid for by the Soviets. Earlier, Andropov had invited the fifth grader to a children's camp in the Soviet Union after she wrote asking why he wanted to "conquer the world or at least the United States."

"She said she didn't like children's camps," Dobrynin said. " 'Okay then,' the secretary-general told her, 'go to Moscow, too, and bring your parents.' "

Then there was a Central American trouble spot, El Salvador, and Salvadoran Ambassador Ernesto Rivas-Gallont praised Reagan's choice of Stone just as the president was giving his nominee a hug. Nancy Reagan stood nearby, wearing a black-and-white dress of oversized polka dots and featuring one bare shoulder.

The ambassadors and their wives milled around an elaborate buffet of roast beef, deep-fried oysters, scallops and smoked salmon. For at least one of them, the evening was a study in how power can be so fleeting.

Dutch Ambassador Jan Hendrik Lubbers said he was No. 120 in the order of protocol two years ago, when he attended his first White House diplomatic reception. Last night he was No. 45.

"If you stand in line and assume that the president takes a half a minute for each ambassador, then the first year, I waited an hour to be presented," Lubbers said. "Tonight, it was 15 minutes."