The euphoria of last summer is gone from the Republican Party. So is the brave talk of taking over the House of Representatives this year.

It has been replaced by scaled-down expectations and nagging fears about what the national economy will do to GOP prospects next fall.

"The future of the party for the next two or three years is tied up in the economy and Ronald Reagan," Larry Forgy, a GOP national committeeman from Kentucky, said last week. "He has taken a helluva gamble; the whole party has taken a gamble."

But the mood of the party is still upbeat, particularly in southern and western states, judging from interviews with a cross-section of party leaders here for the winter meeting of the Republican National Committee.

Reagan stands 10 feet tall in their eyes. The Democrats are in such low repute that state GOP leaders express guarded hope of accomplishing the once-impossible: upsetting such Democratic stalwarts as Senate Minority Leader Robert C. Byrd in West Virginia, Sen. John C. Stennis in Mississippi and Sen. Quentin N. Burdick in North Dakota.

GOP leaders feel they have the candidates, the issues and the money working for them more than at any time in recent memory. Even so, almost every conversation with a state party leader last week ended with a big "if."

"It all depends on the economy," said one border state Republican chairman. "If it picks up we'll do very well. If it doesn't, we're in trouble."

By definition, the state Republican leaders are true believers, not given to potshots at a Republican president. They want to believe that Reagan's economic program will work. Most think it will. The question for them is, When?

"In purely political terms, the upturn has to happen soon. I mean by late spring or early summer," House Minority Leader Robert H. Michel (R-Ill.) said in a speech that captured the mood of the group. "We need it in Congress to hold the troops. We need it in the fall elections to win the kind of support we need."

The White House rolled out some of its heaviest hitters to soothe fears among party field commanders. Their message was simple: don't desert the ship, good news is acoming.

Richard Wirthlin, the president's pollster, said his polls have found the public is willing to give the economic program a chance to work. Almost 60 percent of the public does not expect things to improve for a year, he said.

This was just what many party leaders wanted to hear. "I knew my area was still behind Reagan, but from what I'd been reading in the press I had the impression that we were getting to be an isolated island," said Vermont GOP chairman Charles Coy, a state representative. "They changed that perception."

Not everyone was convinced.

"Nationally, the attitude looks good. I think people will be patient," said one border state chairman. "But I'm not so sure about my state."

The economy has already changed the political equation in many states. The governorships in Michigan and Minnesota, now held by Republicans, are in jeopardy. One year ago, Sen. Donald W. Riegle Jr. (D-Mich.) looked like an easy mark for Republicans. Not any more.

In Minnesota, party leaders worry about keeping two congressional seats now held by Republicans and, as one stalwart put it, "taking a bath" in state legislative races.

The party also has not done as well as it had hoped in redistricting. The 1980 census brought about a shift of 17 House seats from northern states to the Sun Belt, and last year party strategists claimed Republicans would pick up a dozen or more seats through redistricting.

It did not work that way. In California, Democrats drew up a plan, which, if it survives a court challenge, is likely to cost the GOP five House seats. In Illinois, the state Supreme Court ruled in favor of a Democratic plan that could cost the GOP two seats.

Most state GOP leaders appeared more willing to stick with Reagan's economic program than some of their counterparts in Congress. "Most of us expect a turnaround," said Illinois chairman Don Adams. "We're optimistic."

The state party leaders' advice for the president was, stick with what you're doing.

"What people are looking for is consistency," said Nevada GOP chairman Frank J. Fahrenkopf Jr. "I think consistency will work for him."

Republican leaders came to Washington in a good week for Reagan. They were almost all impressed with his State of the Union address. Many were intrigued with his New Federalism proposal.

Party blacks caucused in secret to plot how to get a bigger role in the party and the administration. Some privately complained that few blacks had received jobs in the administration, that the party had ignored blacks in the Virginia gubernatorial election, that the decision to grant tax-exempt status to segregated schools had embarrassed them, and that they were worried about the rightward drift of the party.

But after White House meetings with Reagan and Vice President Bush, this type of criticism subsided.

There was also an undercurrent of unhappiness among women. "I'm convinced the president isn't anti-women, but I believe there's a layer of bureaucracy that is," said former national committee chairman Mary Louise Smith of Iowa. "We don't want to refight the battle over ERA or anything like that, but we want our concerns to be heard."

But again this undercurrent never surfaced publicly.

What did surface was a sense of frustration about how to deal with the Democrats and dissatisfied parts of the population.

"How can we go back to our states and answer the allegation, inaccurate as it may be, that the Republican Party isn't the friend of the senior citizen?" Arizona national committeeman Jack Londen asked in one session.

Later, he said the Arizona Republican Party has been deluged with letters from retired people "who actually believe Ronald Reagan has cut Social Security. No matter how untrue this is, people still believe it, and in politics perception is reality."