It is starting already.

Eighteen months before New Hampshire's voters open the 1980 presidential sweepstakes, the state's political leaders are deeply involved in candidate maneuvering.

The most significant development of this extraordinarily early period of activity is the clear consolidation of moderate Republican support behind George Bush as the main rival to conservative favorite Ronald Reagan.

Meanwhile backers of President Carter are scrambling to shore up his admittedly weak position against what is now taken seriously as a possible challenge from Sen. Edward M. Kennedy (D-Mass.), who is coming here next month to keynote the state Democratic convention.

Among the early starters, Bush has built the strongest base. With at least tacit approval of former president Gerald R. Ford, Bush has signed up most of the key leaders who gave Ford his narrow victory over Reagan in the 1976 Republican primary.

Sen. Bob Dole (R-Kan.), Ford's 1976 running mate, and Rep. Philip M. Crane (R-Ill.), the already declared conservative hopeful, are working the conservative side of the street, but they appear so far to have made only minor incursions into Reagan's 1976 leadership group.

However, New Hampshire Gov. Meldrim Thomson Jr. (R) is putting such strong early pressure on Reagan for personal control of his 1980 campaign that it appears likely that other major Reagan figures from 1976, including his campaign chairman, former governor Hugh Gregg, will be squeezed out of the Reagan campaign and therefore looking for other candidates.

On the Democratic side, the best evidence that the 1980 campaign has begun here has been the sudden flurry of phone calls and visits from Carter's family and aides, bent on what one leading Democrat frankly calls "a rescue mission" for the president.

Carter's stock here appears to be even lower than it is nationally. A poll taken in New Hampshire last month for the Republican National Committee showed Carter's overall approval rating has sunk to 33 percent, with negative judgments on his handling of both foreign policy and domestic issues.

Among New Hampshire Democrats, the poll, taken by Market Opinion Research Corp., said Carter enjoyed only a 44 percent approval rating, and only 22 percent of the Democrats surveyed gave him a favorable rating on domestic issues.

Such polls are one reason why there was a stir of excitement last week when it was announced that Kennedy had accepted an invitation to keynote the state Democratic convention in Manchester on Sept. 30. He had refused many earlier bids from the liberal-dominated state executive committee, and his acceptance is taken as a sign that he is at least testing the waters for a possible challenge to Carter in 1980.

It is also one of many signs that the traditional moratorium on starting presidential politics before the midterm election is out the window for 1980.

State Republican Chairman Gerald P. Carmen will counter Kennedy's appearance by presenting the most publicized young Republican star of 1978, Rep. Jack Kemp (R-N.Y.), as the Republican convention keynoter on Oct. 1.

A bid by Carmen to his Democratic counterpart, Joanne symons, for a jointly sponsored Kennedy-Kemp debate on tax policy was rejected.

But when the White House learned of Kennedy's plans, it let local Carter supporters know that the president's son, Chip, will be in New Hampshire for several days that same week as part of his new job as a traveling political ambassador for the Democratic National Committee.

Elliot L. Richardson, a sentimental favorite of some liberal Republicans, will also be speaking that week to the Concord Chamber of Commerce. A bit later, his former Cabinet colleague, John B. Connally, will address the Nashua man-of-the-year banquet.

Even this weekend, while most of the state vacationed, there was no letup in politics. On Friday, Federal Trade Commissioner Elizabeth Hanford Dole spoke to the New Hampshire Council on the Aging, then joined her husband, Bob, who has already made five visits to the state in pursuit of his presidential hopes.

At the same hour she was speaking here, Tim Kraft, Carter's White House political lieutenant and 1976 presidential primaries strategist, was up the students assembled by former state Democratic chairman Larry Radway, a Dartmouth College professor.

All this early action is viewed by some observers as putting a serious crimp in the plans of those potential 1980 presidential candidates who are tied down in home-state campaigns of their own this year.

Such Republicans as Tennessee Sen. Howard H. Baker Jr. and Illinois Gov. James R. Thompson and such Democrats as California Gov. Edmund G. (Jerry) Brown Jr., may find their potential supporters already committed to other campaigns by the time they get here.

Local observers cite at least three reasons for the unusually early start on presidential politics. For one thing, Carter who began visiting New Hampshire in 1974 and assembled his organization in 1975, showed the value of early cultivation, and others are following his example.

Second, there is little in the 1978 races to excite the political buffs. Thomson is seeking his fourth term as governor, and Democrat Thomas J. McIntyre is seeking his third term as senator. Polls show that both men are potentially vulnerable, and both have nominal opposition in the Sept. 12 primary.

But the field of challengers for the general election -- two Democrats are running against Thomson and four Republicans against McIntyre -- includes no one who appears to have excited his own party.

Finally, there is a feeling in the state that 1980 may be the last hurrah for the New Hampshire primary and, as one person said. "We want to milk it for all we can get."

Since 1952, no one has been elected president without first winning the New Hampshire primary. Its first-in-the-nation status has brought dozens of candidates, hundreds of reporters and a rich flow of dollars to the state.

But the disproportionate influence exercised by the voters of this small state has drawn criticism elsewhere. The new Democratic Party rules are designed to shorten the primary season.

New Hampshire has been given a one-time-only exemption for 1980 from the Democrats' ban on any primary being held before the first Tuesday in March. And many here believe that next time is the last time New Hampshire will be allowed to launch the primaries a week ahead of any other state.

If that is true, New Hampshire would go out with a bang.

On the Republican side, the most surprising development is the early leadership strength amassed by Bush, the former Texas congressman, national GOP chairman, ambassador and CIA director, who is expected to announce his candidacy early next year.

Bush visited New Hampshire last May for two speeches and a reception and did so well by all accounts, that he converted the 1976 Ford campaign leaders -- many of them old personal and political friends of his own -- into the nucleus of the 1980 Bush organization.

Among those who indicate they are already actively involved with Bush are Rep. James C. Cleveland (R-N.H.), the 1976 Ford chairman, national committee members Robert P. Bass Jr. and Victoria Zachos, and state government officials Malcolm and Susan McLane, all leaders of the moderate Republican group opposed to Thomson and his conservative political ally, Manchester Union-Leader Publisher William Loeb.

These Bush campaign sponsors say they have had absolutely no indication from Ford that he wishes they would stay loose until he firms up his plans for 1980. Ford is not scheduled to campaign in New Hampshire this fall, while Bush is returning for a three-day swing in October. This strengthens the impression that Ford is deliberately giving Bush wide running room for a fullscale try to beat Reagan in the 1980 kickoff primary.

Recognizing this Thomas, and other Reagan backers are already zeroing in on Bush. In an interview, the governor described Bush as a "recognizable liberal. He's a nice fellow," he said, "but he's an extension of Rockefeller-type politics, and he won't get anywhere here."

Others in the party leadership say, however, that Bush's CIA experience and his strong stand on national defense -- both stressed in his May speeches here -- won applause from conservatives.

Gregg, the 1976 Reagan chairman, and Stewart Lamprey, the party pro whose computerized voter list was used effectively by the Reagan forces in 1976, said in separate interviews last week that Bush has positioned himself as the best bet to win in a multicandidate field including Reagan and other conservatives.

"He would certainly have the best chance if there are more than two candidates," Gregg said. Lamprey described Bush as "clearly the frontrunner among those who have a strong interest in politics."

With Crane already announced and Dole moving closer to an official candidacy, both Thomson and Loeb have been warning conservatives of the danger of a divided vote defeating Reagan.

So far, the warnings have fallen on deaf ears. Crane has been up here to campaign for conservative candidates in the primaries for the House and Senate, and has won support from business association executive Dick Flynn, a newcomer to New Hampshire with few loyalties to Thomson or Loeb.

Dole has spent 10 days in the state, looking up World War II buddies from the 10th Mountain Division and dropping in for personal chats with workers from both the Ford and Reagan campaigns. He has reestablished some ties to his divorced first wife, a native of New Hampshire, and has let it be known that he is on friendly terms with his former mother-in-law who lives in Concord.

Dole has enjoyed at least tacit approval from state GOP chairman Carmen, a Thomson ally, who has allowed his top staff aide to assist Dole in scheduling his trips.

But, in an interview, Thomson severely criticized Dole's support last week for the constitutional amendment to allow voting representation in Congress for the District of Columbia. The governor said this would "do nothing but increase the power of the welfare recipients and bureaucrats who live in Washington. I have told Dole," he said, "it will hurt him badly up here, because his was the key vote in passing that amendment. He told me he supported it because it was in the Republican platform, but that is a flimsly excuse."

That kind of ideological rigidity is characteristic of the governor, and is a potential problem for his candidate, Reagan. In 1976, Reagan kept a certain distance from Thomson, giving control of the campaign to Gregg, a more moderate conservative. Thomson made it clear in the interview he does not want to allow that to happen again in 1980.

"Last time," he said, "he [Reagan] blew it in New Hampshire. He could have won it, but he had some weak fellows around him . . . people who wanted him to be a nice guy and not take any strong stands. He'll blow it again next time if he follows their advice."

The only individual Thomson singled out for criticism was John Sears, Reagan's 1976 national campaign manager. But close associates said the governor is deliberately holding open the option of running as a favorite-son if Reagan hesitates to give him full control of the 1980 campaign. The issue is likely to be settled when Reagan comes here in October to speak for Thomson.

In any case, Gregg seems to be out of the Reagan picture for 1980. The former governor said that Reagan aides were upset when he declined to join the board of Reagan's interim political action group, Citizens for the Republic, and he has not had contact from Reagan in recent months.

"Others tell me he is going to announce in January or February," Gregg said, "so I assume that the people who have been told of his plans --Thomson and Carmen and Loeb -- will run his campaign next time."

On the Democratic side, the last few weeks have seen a sudden effort by the White House to begin fencemending here. Just before she left on the family vacation in the West, Rosalynn Carter called Jean Walin an early Carter supporter in Nashua, "just to chat and see how things were," and to tell her she planned to visit New Hampshire this fall.

Walin said she told the First Lady that "there is a general kind of wishy-washiness" toward Carter but "no real animosity. The professional politicians are ticked off because he's left Republicans in some jobs here, and the man in the street says, "Tell me what he's done right."

Lucille Kelly, the Manchester politician who accompanied Carter on his first tour of the state, is even more worried about his situation.

"I think the world of the presiden," she said, "but there is a general feeling of disappointment. There is definitely something lacking there. He has a problem with some of his closest advisers -- a lack of experience as far as the Washington scene goes and what is the most expedient way to deal with Congress."

Another early Carter backer, now occupying a federal appointive post, said he thought the voters would come to see that the president "is really giving it his best shot, and if there's a problem, it's with Congress, not Carter."

But backers of Carter's main rivals here in 1976, Rep. Morris K. Udall (D-Ariz.) and Sen. Birch Bay (D-Ind.), many of whom still occupy key posts in state politics, say the attitude toward Carter ranges from embarrassment to hostility.

These Democrats are readily available recruits for a Kennedy campaign. Even Lucille Kelly said, "If he runs, I'd definitely say that Ted has the top shot in New Hampshire. The name is magic here."