A pessimist was as scarce as a socialist at the Republican National Committee's meeting yesterday.

There were a few skeptics who thought the celebrating might be a bit premature, as the GOP leaders gathered in the Mayflower Hotel for the last time before the national convention meets in Detroit in July for what most of them believe will be the nomination of Ronald Reagan.

But euphoria was the preailing tone, which was a bit of a surprise, because George Bush, Reagan's struggling challenger, was the early favorite of the party establishmentarians.

"It could be a dynamite year," said Bruce Melchert, the Indiana GOP chairman. "The climate is there for a real change."

The same thought was echoed by Repulicans as far removed from each other geographically and politically as Lucie Humphrey, the Nevada committeewoman, and Dr. Bernard M. Kilbourn, the New York chairman.

Having Reagan and Paul [Laxalt] at the top of our ballot will help everyone," Humphrey said of the Nevada prospects for the prospective nominee and the state's incumbent senator.

"We'll have a good year at the state level, and I really think Reagan can carry New York," Kilbourn echoed.

The words the officials heard from the podium were relentlessly upbeat in tone, starting with national chairman Bill Brock's declaration that "it's sort of fun to be a Republican these days."

Finance Chairman Joe M. Rodgers said the party now has 1 million contributors, and Brock cited a poll showing that the proportion of voters calling themselves Republicans had grown from 24 to 30 percent in the past three months.

In the face of all this, it was hard to find anyone who was overly gloomy, but there were some veterans of the political wars who suggested that it might be well to wait for more votes to be counted.

"I remind the Reagan people in our state," said Ody Fish, the Wisconsin committeeman, "that Reagan got 22 percent of the total vote that was cast in our Republican and Democratic primary. We have a reasonable opportunity to win, but we also have a chance to paint ourselves into a corner, if they don't try to broaden the appeal."

"I think the optimism is based more on how bad Carter has been than any great enthusiasm for our alternative," said Keith L. Brown of Colorado. "We better not blow it. When we get to Detroit, the unemployment there may be about 30 percent. It's not a time for us to be blowing horns and celebrating; we ought to be a lot more serious."

Even Melchert, the Indiana chairman, cautioned that it would be a mistake "to get smug and think the Democrats are going to do it themselves. The third candidate running in this race is complacency, and that's a legitimate concern for us."

John P. East, the North Carolina committeeman and the GOP candidate for the Senate, is an early Reagan supporter who finds the prospect of running with him delightful. "The trends in national politics are clearly moving our way," he said.

But Bush, the former party chairman, began 1980 with more friends and backers on the national committee than any of his rivals, and some of those members were unhappy at what they called "pressure" on them to urge Bush to end his challenge to Reagan.

A resolution to that effect was discussed at a breakfast meeting of Northeast GOP leaders, but was hastily withdrawn when Bush supporters from New Hampshire, Rhode Island, Connecticut and Pennsylvania vowed a fight.

Many of the Bush partisans are lame ducks on the committee, awaiting replacement by their pro-Reagan convention delegations.

But there was some concern expressed about how far the takeover might go. Vern Neppl, the Minnesota GOP chairman, who endorsed Reagan just last week, said he was urging the state's Reagan leaders "not to be so damn strident and greedy" in their approach.

He said he was unhappy that the Reagan forces had balked at giving a delegate seat to Gov. Al Quie (R) until Quie last weekend endorsed Reagan, and were still threatening to keep Sen. David Durenberger R-Minn.) off the delegation because he is neutral.

One of the party's most prestigious elder statemen, retiring from the committee this year, expressed himself in a not-for-attribution comment: "I can't understand this feeling that we've got it wrapped up. Let's face it, Ronald Reagan isn't the ideal candidate. He's 69 years old. I'm 72, and I know enough to step down from this job. Why doesn't he know it? That job is a man-killer. You tell me that people aren't going to worry about that?"