Republican Party leaders, flush with cash and confidence, are moving systematically to exploit what many of them consider a historic opportunity to become the nation's majority party. But they concede that looming economic problems and an early-starting fight for succession to Ronald Reagan could cost them a breakthrough.

The contrast in mood between the upbeat Republican National Committee meeting that concluded here today and the tight-lipped Democratic National Committee gathering in Washington earlier last week was as great as the gap in last year's presidential results.

In a revival-meeting atmosphere in their ultramodern skyscraper hotel, Republicans welcomed into their midst a Louisiana legislator who resigned as a member of the Democratic National Committee last Monday, and six current and former Georgia Democratic officials who jumped parties Friday.

"A Republican tidal wave is sweeping the nation," party Chairman Frank J. Fahrenkopf Jr. said.

In their far less glamorous Washington hotel ballroom, Democrats dismissed the prominent party-switchers in Texas, Michigan, Massachusetts, North Carolina and other states as ingrates and opportunists. They cheered as House Budget Committee Chairman William H. Gray III said Republicanism hit its "high-water mark" with Reagan's reelection and predicted it will fall victim to rising budget and trade deficits.

Some Republicans say they are afraid the Democrats may be right. Presidential pollster Richard B. Wirthlin's report to the RNC was overwhelmingly optimistic, but confirmed that the percentages of Americans who believe the country is on the right course and approve of Reagan's leadership, while still healthy, are down from 1984 highs by 18 and 9 points, respectively.

The public mood is "full of promise" for the GOP, Wirthlin said, but also "full of challenge." He noted that the current economic recovery will be 33 months old in September, the terminal point for the average growth cycle since World War II.

Farm woes worry Midwest Republicans.

"The farmers are very, very patient now, and they're not blaming anybody," veteran Kansas Republican National Committee member McDill (Huck) Boyd said. "But another year of hammering, they'll be ready to retaliate. Between the Dakotas and Oklahoma, we've got a lot of senators who could get hurt -- and in the South, as well."

Georgia Republican National Committee member Carl L. Gillis Jr. said the prospect of annual $200 billion deficits is so scary that "personally, I'd just as soon let the Democrats have it [the presidency] in 1988. I'm just afraid that, if we're in power when it [the crash] comes, we may be dead for 25 years."

Whatever the future, today's picture is the most favorable in decades for the GOP, which has matured into an affluent, effective and aggressive political merchandising mechanism, at a moment when the Democrats see themselves struggling for credibility and solvency.

The disparity in the financial and organizational resources of the two parties is no longer a new story, but its consequences cannot be exaggerated. The Republican National Committee has raised five times as much as its Democratic counterpart this year and shows no sign of slowing its pace. Even more striking, and probably of greater impact in upcoming contests, is the gap between the parties at the state level.

In New Jersey, the site of one of this year's key gubernatorial and legislative contests, the Republican state committee has a budget of more than $2 million, no debt and an 11-person staff, including five field organizers. The Democratic state committee hopes to raise million, but has a $550,000 debt and a staff consisting of an executive director and a secretary.

In three 1986 battleground states surveyed by The Post, the GOP staffing and finance advantages range from modest in Iowa and Texas to glaring in Illinois. The Illinois Republican state committee has a $1 million budget for 1985 and employs a dozen people, while Democrats hope to raise $100,000 to pay a staff of four. Their executive director moonlights from her job on the chairman's legislative payroll.

Democrats receive help in some states from city and county political machines and from organized labor, but no longer can count on a "natural majority" of voters to cushion the impact of superior GOP communication, organization and voter-turnout techniques.

In the newest Washington Post-ABC News Poll of1,506 registered voters, completed eight days ago,48 percent said they considered themselves or leaned to the Democrats, 45 percent Republicans. Wirthlin, who found an almost identical 44-to-41 percent margin in his June polling, pointed out that as recently as August 1983 Democrats enjoyed a 19-point advantage.

This is not the first time Republicans have almost drawn abreast in that basic measure of party identification. They did so in 1981, after Reagan's first victory, then saw the converts become disaffected during the recession. This time, they are trying to attract Democratic officeholders and voters to the GOP column while the GOP is hot.

But, as GOP pollster V. Lance Tarrance pointed out , "there is still a question whether this shift in voter identification is a force in itself or whether it's tied so much to Reagan it may not outlast his presidency."

It was evident at the RNC meeting that party leaders are feeling the pressure of the search for a successor.

Texas GOP Chairman George W. Strake, a Houston neighbor of Vice President Bush but a philosophical ally of Rep. Jack Kemp (R-N.Y.), said: "I don't want to face that choice. After '86, I'm tempted to take a two-year hunting trip to Alaska."

Michigan National Committee member Peter F. Secchia said the recent Midwest GOP conference "got people thinking so much about the 1988 fight they've forgotten we have a governor's race next year."