If President Reagan decides not to run for reelection, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) has a contingency plan ready in his desk. Media consultant Douglas Bailey prepared the 50-page document about a month ago.

The booklet is a blueprint for the first month of a Baker campaign for the 1984 Republican presidential nomination. It includes a checklist of what Baker would have to do from the moment word came from the White House: the calls to make, the bases to touch, what to tell reporters.

The idea is to give Baker a leg up on Vice President Bush, Sen. Robert J. Dole (Kan.), Rep. Jack Kemp (N.Y.), Sen. Paul Laxalt (Nev.) or any other Republican presidential hopeful, should the unexpected occur.

"I call it my '1 Percent Plan,' " Baker said. "I don't think we'll ever need it. I've urged the president to run for reelection and I fully expect him to run. But in the 1 percent chance he decides not to, we're ready."

Reagan political operatives have been so successful in convincing party leaders that the president will seek a second term that no major Republican figure has made any visible effort to put together a campaign organization, or appears overly eager to succeed the president.

"They're all on hold," said Laxalt, general chairman of the Republican Party. "I've told them all we have a candidate."

"Most everyone I know has decided there's no vacancy," Dole agreed. "I haven't seen any moving vans around the White House."

Only one Republican, Sen. Bob Packwood, a moderate from Oregon, has openly challenged the president's leadership and policies in campaign-like trips to New England and Iowa. And he insists he wouldn't run for president under any circumstances.

But Dole, Baker, Kemp and Bush have made subtle--and perhaps unintentional--moves that could enhance their prospects as presidential hopefuls.

These moves may accelerate with the announcement that Reagan has canceled a major meeting with political advisers on Aug. 21, where his reelection campaign was to have been discussed.

Reagan, after all, is at age 72 the oldest incumbent president in U.S. history, and until he announces his decision there will be an element of uncertainty.

Baker positioned himself early for an all-out dash for the White House last winter by announcing he will not seek reelection to the Senate.

Baker, Kemp and Dole have mounted efforts to increase the sizes of their political action committees. Such committees in the past have been used to launch the presidential candidacies of Reagan, Bush and Democrat Walter F. Mondale, among others.

Baker's Republican Majority Fund PAC raised $954,227, a new high for a nonelection period, during the first six months of the year, according to Federal Election Commission reports.

After sinking much of that money into direct-mail activities to attract future donors, the PAC had $157,719 on hand June 30. Baker could use the PAC's extensive list to raise money quickly for a presidential race.

Dole's Campaign America PAC, which he used to get his unsuccessful 1980 campaign off the ground, raised $342,305 during the first half of the year, and had $203,115 on hand June 30, according to FEC records.

Dole, according to Iowa political sources, has also been a frequent visitor to that state, where the first formal test of the 1984 political season will be held.

Kemp's Campaign for Prosperity PAC has upset Reagan political operatives by sending fund-raising letters that they think make the congressman sound too much like a presidential candidate.

The committee has not filed its FEC reports, but has launched an effort to raise twice as much money for the next election cycle as the last, a Kemp spokesman said. The PAC gave $105,600 to GOP candidates in 1982, according to FEC records.

Bush closed his PAC when he became vice president, and he has warned his staff members that he would fire anyone who does anything that could be interpreted as leading to an '84 presidential candidacy, according to press secretary Shirley Green.

But as part of his job he maintains an almost campaign-like schedule, meeting with political leaders and businessmen across the country who could be called on for help later.

He has courted the Rev. Jerry Falwell, leader of the Moral Majority, and other conservatives suspicious of his reputation as a moderate. He also built up scores of political IOUs in 1982 by traveling 314,000 miles during which he made appearances for 120 candidates and raised $28.7 million.

Kemp, Baker, and Dole also speak frequently at party functions, but spokesmen for the three say they haven't added to their schedules to promote any presidential aspirations this year.

Kemp is so sensitive to any hint of disloyalty to the president that he turned down an invitation to address a statewide GOP fund-raising dinner in Iowa last April. Baker went instead.

"You're automatically suspect of undercutting the president if you go to Iowa or New Hampshire so we're just staying away," said Kemp's administrative assistant, David Smick.

Packwood has been the only Republican noncandidate to openly thumb his nose at the White House by venturing into early primary and caucus states. He has attracted large and enthusiastic crowds during three trips delivering tough anti-Reagan messages. Last month in Des Moines, for example, he said:

"The president had an extraordinary opportunity to make the Republican Party a majority party. He could have built a solid blue-collar base. He could have built a solid women's base. He could have changed the alignment of American politics but we've lost a golden opportunity."

You won't hear that kind of talk from Baker, Bush, Dole, Kemp or Laxalt, who is mentioned by some conservatives as a potential candidate, something he has tried to discourage as general chairman of the party.

They are seasoned professional politicians, waiting for the moment in the wings. They want to protect their options, and not alienate Reagan and his supporters by appearing opportunistic or disloyal. They all appear convinced that Reagan will be a candidate.

Reagan advisers have indicated the president may not announce his decision until mid-November. Charles Black, a leading GOP political consultant, said the first official deadline any candidate would have to meet doesn't come until early January when delegate slates have to be filed in Illinois.

But there are dozens of other practical considerations to be met earlier, such as putting together a staff, raising money, qualifying for federal matching funds, and building campaign organizations in key states.

"If Reagan decides not to run, all of us are going to be up the creek in a wire canoe," complained Smick, Kemp's aide.

Bush's canoe, party professionals agree, would start out with clear advantages. He has endeared himself to Reagan by being a loyal and self-effacing team player, and it is widely assumed in the White House that he currently is Reagan's choice as a successor.

The vice president has a considerable following of his own. If Reagan isn't a candidate, 52 percent of those surveyed in a June Washington Post-ABC poll said they'd vote for Bush; 25 percent for Baker; 10 percent for Dole, and 7 percent for Kemp.

Bush built up an impressive political organization in his unsuccessful 1980 bid, and could call on many old supporters for another race. Almost every time Iowa Republicans have gathered for an official function for the last 2 1/2 years, the leaders of the old Bush campaign, for example, have found a reason to get together either before or after the meeting.

They have been primarily social functions, said Ralph Brown, one of Bush's first 1980 supporters. "But if something were to happen there would be people we could talk to and get moving very fast."

Baker is well aware that he'd have to move fast also. That's why he has his "1 Percent Plan" prepared.

"If Reagan makes an announcement, he has five or 10 minutes to make up his mind what to do," said media consultant Bailey. "He should leave no doubt in anyone's mind about his decision."