While Ronald Reagan contests for a presidential nomination that seems firmly in his grasp, the central focus of his campaign has shifted to the selection of a running mate.

As Reagan returns to the road for his last cross-country campaign tour of the primaries, his pollster Richard B. Wirthlin is preparing to survey voters nationwide to find out who they think the runnng mate should be.

Other Reagan aides are interviewing prominent Republicans, an effort they hope will take some of the steam out of the lobbying now developing for and against the two presumed top vice presidential candidates -- Sen. Howard H. Baker Jr. of Tennessee, and Rep. Jack Kemp of New York.

Reagan's choice is considered so highly critical that some of his aides acknowledge it could spell the difference between victory and defeat in November. It is a view widely shared in the Republican Party, where the vice presidential selection is likely to be viewed as symbolic of the way Reagan would behave in the White House.

Wirthlin's survey will contain a list of 15 to 18 names, most of them of well-known Republicans. It will try to measure the attributes voters desire in a vice president, particularly one who would serve with a president who would be 70 years old a month after inauguration. And it will provide clues about the acceptability of prospective nominees on the basis of age, ideology, region, religion and sex.

"This is going to be Gov. Reagan's decision, not his staff's," said Wirthlin, Reagan's chief strategist. "But he's going to have the widest possible information base he needs to make it. The decision isn't going to be based on whim or whimsy amidst the adverse circumstances of the morning-after nomination."

While the surveying is going on, Reagan chief of staff Edwin Meese and campaign director William J. Casey will interview a number of prominent Republicans and others respected by the candidate for their views. Meese said that this process, along with the polling, should be completed by mid-June.

The very nature of the process could lead to a relatively moderate vice presidential nominee. GOP leaders from various sections of the country who were interviewed during last week's meeting of the Republican National Committee generally expressed the view that Reagan should try to balance the ticket by selecting a running mate who appeals to different constituents than he does.

"I don't like to use the word "balance' because balance means different things to different people, but I would like to see a ticket that has appeal to every American in every walk of life," said Republican National Chairman Bill Brock, himself considered in some quarters as a longshot possibility for vice president.

A Reagan aide who respects both Brock and Baker agreed, though he pointed out that Reagan may be constrained by past commitments to nominate someone who shares his philosophical views.

"Nevertheless, this is a campaign that smells victory and wants to win," said the aide. "By making the choice objectively, with polling data and lots of opinions, Gov. Reagan will be able to say that he picked the person most acceptable to the party, or the nation."

While there is no formal list, the same names arise frequently in any discussion of the vice presidency within the Reagan camp. Heading the group are four Republicans, factiously referred to by one aide as "the gang of four."

These four are Baker, Kemp. Sen. Richard G. Lugar of Indiana and former treasury secretary William E. Simon. Others whose names are believed to be on the survey list include Brock, Illinois Gov. James R. Thompson, Texas Gov. Bill Clements, former treasury secreatry John B. Connally, Sen. Paul Laxalt of Nevada and former United Nations ambassador Geroge Bush, Reagan's sole surviving GOP opponent.

No one in the Reagan camp can be found who expects Bush to be Reagan's running mate. Reagan and his aides are concerned about how a running mate would fare in debates with Vice President Mondale, a respected adversary. Baker and Kemp are considered good debaters, while Bush has proven ineffective in a series of debates with Reagan.

Here is the early handicapping on the four supposedly to contenders:

Baker, the 55-year-old Senate minority leader, is the favorite of GOP moderates and reportedly has support from key Reagan financial backers. He would be viewed as "presidential" and his knowledge of foreign policy is a plus. He also is considered helpful in the South, where Reagan hopes to contest favorite son Presidential Carter. But Baker has two strikes against him -- his support of the Panama Canal treaties, anathema to conservatives, and his opposition to a constitutional amendment restricting abortion to cases necessary to save a woman's life. Reagan favors the amendment and has said he wants someone on the ticket who agrees with him on this issue.

Kemp, a 44-year-old fifth-term congressman from Buffalo, has important support from the "New Right" and from advocates of the tax-cutting policy, which bears his name in the Kemp-Roth bill, a cornerstone of Reagan's campaign. Kemp also agrees with Reagan's campaign. Kemp also agrees with Reagan on the abortion issue and comes from a state which Reagan aides believe will be a crucial battleground in November. But some Republicans consider Kemp too strident and he has the opposition of traditionalist conservatives who stress budget-balancing over tax-cutting.

Lugar, a 48-year-old first-term senator from Indiana and former mayor of Indianapolis, is thought by many to be the perfectly positioned dark horse if Baker and Kemp cancel each other out. Because he was against the Panama Canal treaties and supports the anti-abortion amendement, he is far more acceptable than Baker to the conservatives. And he is less controversial than Kemp, although also less well known.

Simon, 52, now a senior consultant for a New York brokerage firm, is the only Catholic on the list of four. This is considered an asset because of Reagan's expected appeal (in a race against Carter) to Catholic voters. But Simon, a father of seven who personally opposes abortion, does not support the antiabortion amendment, saying, "I don't like tinkering with the Constitution and neither did the founding fathers, who made it difficult to amend." Simon has support from traditionalist economists, and he is highly thought of personally by Reagan. However, he lacks a base of support within, the party comparable to that of Baker, Lugar or Kemp.

Reagan's aides are aware that the candidate will be under heavy pressure until he makes his vice presidential selection. Meese said he hopes this pressure can be diffused by giving everyone who has an opinion an opportunity to be heard and by convincing them that the choice will be made in a serious fashion.

"Gov. Reagan views this as a very critical choice and he isn't going to take it lightly," Meese said.