There is an old saying that U.S. senators are transformed from world statesmen to precinct politicians two years before their next election. If that is so, the time of metamorphosis is at hand for an exceptionally large and powerful group of Republicans.

The roster of the class of 1984 features a mix of Washington clout and home-state vulnerability unprecedented in the last quarter century.

Within the ranks of the 19 Republicans facing reelection next year are the majority leader, majority whip and chairmen of most of the Senate's major committees--Budget, Appropriations, Armed Services, Agriculture, Judiciary, Foreign Relations, Energy and Natural Resources and Veterans' Affairs. Still, 10 of the 19 are only first-termers, and an equal number won in 1978 with less than safe margins.

Such an array of Republican senators has not been exposed to the electorate since 1958, when the results were disastrous for the party. Nine of the 19 Republican incumbents lost that year, while all 14 incumbent Democrats won.

The fear that a defeat approaching that magnitude--one that would return Democrats to power--could be repeated next year might be among the most significant and unpredictable factors in the Republican-controlled Senate during the 98th Congress. It also helps explain why party leaders have been making so many trips to the Reagan White House in an effort to fashion policies they think will be more acceptable to the voters.

"Our class of '84 will be in a unique position. They'll be pulled in three directions at once," said one Republican adviser in the Senate.

"First they've got to deal with the reality that solutions to the major issues of the economy and Social Security go beyond partisanship and local concerns.

"And they've got to deal with a president of the same party whose interests are not necessarily theirs. And they've got to satisfy the home folks to get reelected and stay in power," he said.

Republican senators and party strategists interviewed last week were not at all unanimous about the best way to accomplish that considerable task.

Some said they would continue the recent trend of establishing independence from the Reagan administration and then make their reelection contests as state-oriented and decentralized as possible.

Said Larry Pressler (S.D.): "If Reagan runs again, it will be a solo thing, the White House will protect itself, and the rest of us will be scrambling as best we can. I've pretty much taken my own approach to things since I got here, and I'll continue doing that."

But Richard G. Lugar (Ind.), chairman of the National Republican Senatorial Committee, said he has arranged a series of meetings with the 19 incumbents over the next year in the hope that the group will develop a united strategy to retain power.

"There has to be sharing, rather than every man for himself. That suggests a strategy of panic," said Lugar, whose defeat of Bob Packwood (Ore.) for the campaign committee post was interpreted as a victory for Reagan forces in the party.

" . . . The thing I'm trying to emphasize is that those of us who survived in '82 were well organized and very well funded and won because of a great amount of tedious work," Lugar said. "And there's no time like now to get ready for it."

One of the first questions Lugar faces in getting ready for 1984 is how many of the incumbent Republicans will decide to run again.

Howard H. Baker Jr. (Tenn.), the respected majority leader who has presidential ambitions, let it be known last week that he might leave the Senate at the end of this term.

Baker said he would reveal his plans before the end of January. There has been some uncertainty about the intentions of a few other heavyweight senators, including Strom Thurmond (S.C.), John G. Tower (Tex.) and Mark O. Hatfield (Ore.), but all three have been raising money and have intensified their campaign schedules back home.

Speculation about the possible retirement of Hatfield, the liberal chairman of the Appropriations Committee, had been most intense in recent months, but his aides attributed it to a newspaper article in which their boss said he did not want to "mummify" in the Senate.

"The senator was just waxing philosophical and the story stretched the point," one aide said.

"We're not sure what his plans are, but there has been no change whatsoever in the frequency of his trips home and we take that as a positive sign."

Should he run again, Hatfield would be classified in the "safe" category by strategists in both parties. Other Republican incumbents who at this point appear invulnerable to a Democratic challenge include:

William S. Cohen (Maine), who garnered 57 percent of the vote in defeating incumbent Democratic Sen. William Hathaway in 1978, is an articulate moderate-liberal in a state that rewards independence. "The election of '82 made the world safe for Bill Cohen," one Democratic party strategist said. "He will not walk the plank with Reagan."

Nancy Landon Kassebaum (Kan.), another moderate first-termer, who came here known primarily as the daughter of former governor Alf Landon. She has gained her colleagues' respect and appears so popular in Kansas that a Democratic campaign consultant said he would "hate to run a campaign against her."

James A. McClure (Idaho), chairman of the Energy and Natural Resources Committee, who comes from one of the few states where the Republicans dominate politics on the local, state and national level. He won with 68 percent of the vote last time, and the political wisdom now is that the only Democrat who could give him a reasonably close race is former Democratic governor Cecil Andrus.

Alan K. Simpson (Wyo.), who, like McClure, is a conservative who has supported all of Reagan's economic policies and yet has an image as a western independent on energy issues. In his first term, he is chairman of the Veterans' Affairs Committee. "He is the quintessential Wyoming senator," a state Democrat said. "It's going to take one helluva candidate for us to come within shouting distance of him."

Republican incumbents who appear to be in the most danger include:

Jesse Helms (N.C.), chairman of the Agriculture Committee and leader of the New Right faction of the party, who appears to face the race of his life in 1984 against Democratic Gov. James B. Hunt, a comfortable leader in early polls. Their face-off is expected to be one of the most heated and expensive in the country.

Gordon J. Humphrey (N.H.), who defeated incumbent Democratic Sen. Thomas J. McIntyre by only 6,000 votes in 1978 and has been one of Reagan's loyal followers in the Senate.

He irritated the White House, and many of his constituents, by joining Helms' filibuster of the gasoline tax-highways bill during the lame-duck session, but backed off at the end under considerable pressure from his colleagues.

"Every Democrat in the state wants to run against him," one campaign consultant said.

Polls have shown that environmental issues are the top concern of New England voters, and Humphrey's record in that area has been modest. Recently he has attempted to recover some of the "green vote" by becoming active on the issue of acid rain.

Thad Cochran (Miss.), who won with only 45 percent of the vote in 1978 as Charles Evers sapped Democratic strength with his third-party campaign. The racial polarity that has historically plagued Mississippi's Democratic Party virtually disappeared in 1982, however, as Gov. William F. Winter campaigned hard for black candidates all over the state.

Cochran, whose state has more food stamp recipients per capita than any in the nation, opposed many of Reagan's social service cuts and promises to do so again, but it might not be enough, especially if Winter runs against him. Said one Democratic party consultant: "You give me a united Democratic party in Mississippi, and I'll give you a Democratic senator."

Roger W. Jepsen (Iowa), a member of the New Right faction who defeated incumbent Democratic Sen. Dick Clark by a narrow margin in 1978. His state party is badly divided between moderates and conservatives. He is considered one of the lesser lights of the Senate and has been targeted for defeat by national Democrats.

John G. Tower (Tex.), chairman of the Armed Services Committee, who has been in the Senate since 1961 but whose victories have never been easy. In 1978, he defeated former representative Robert Krueger by only 12,000 votes, even though that was a big year for state Republicans as Bill Clements won the governorship.

Clements was defeated last November and his party virtually obliterated state-wide, making it that much harder for Tower this time around, with Krueger and several other Democrats lining up for the challenge.

"Texas might be less publicized than North Carolina right now, but the race there will be even more heated and expensive," Lugar said. "John is hard at work pulling together the coalition that has pulled him through close races. Our work is cut out for us in winning that race again."

Rudy Boschwitz (Minn.), a first-termer who handily defeated former governor and senator Wendell Anderson in 1978. His moderate Republican colleague, Sen. David F. Durenberger, barely survived a challenge in 1982, and Democrats believe that Boschwitz, a wealthy and flamboyant politician, can be beaten.

"Durenberger may have survived because the voters were turned off by the big money of his opponent," one campaign consultant said. "That won't be an issue with the millionaire Boschwitz."

Charles H. Percy (Ill.), chairman of the Foreign Relations Committee, who has been in the Senate since 1966 but won with only 53 percent of the vote in 1978. He will probably face a challenge in the Republican primary from conservatives, who dislike his breaks with the Reagan administration over Middle East policies. "The yahoos might beat him," a Democratic consultant said. "And if they don't, they'll weaken him enough so we'll have a chance to finish him off."

In the middle of the scale are seven Republican incumbents who would be considered favorites to win reelection unless the economy worsens and Democrats score a sweep of landslide proportions. They include:

Howard H. Baker Jr. (Tenn.), the majority leader, who has never piled up big wins in his home state. If he runs again, he might be challenged by popular Rep. Albert Gore Jr.

Ted Stevens (Alaska), the majority whip, who won with a massive 76 percent of the vote in 1978 but is known as a somewhat erratic politician who could face a difficult race if a strong challenger emerges.

Strom Thurmond (S.C.), chairman of the Judiciary Committee, who is 80, which alone might make him vulnerable to a challenge from Gov. Richard W. Riley. As was the case in many states in 1982, the state's Republican party was leveled by the November elections.

Pete V. Domenici (N.M.), chairman of the Budget Committee, who faces a dilemma much like that of Baker. He is extremely well regarded here but has had tough races back home. In his last race, he won only 53 percent of the vote. National campaign experts believe that the state's Democratic Party was perhaps the nation's most unified and effective during the 1982 campaigns.

William L. Armstrong (Colo.), who, though a first-termer, has become one of the Senate's most effective conservatives. He is in good standing in Colorado, but might have difficulty in 1984 because of the caliber of his possible opponents, who include Gov. Richard D. Lamm and Reps. Timothy E. Wirth and Patricia Schroeder.

Larry Pressler (S.D.), who during his first term has been criticized by some GOP colleagues for a tendency to seek publicity rather than solutions. The Democrats think they could beat him with a strong candidate, but there appear to be none around.

John W. Warner (Va.), who had the closest race of all in 1978, winning by only 5,000 votes, but has made few enemies during his first term. Democrats have a paucity of strong candidates to oppose him. They believe Gov. Charles Robb could do it, but Robb, who has national ambitions, has said that he will stay in Richmond.