When Ronald Reagan launched his campaign for the presidency, campaign manager John P. Sears was considered the resident mastermind of Republican politics, the supreme strategist who nearly pulled off the nomination for Reagan last time and was a sure bet to do it in 1980.

Now, before the first primary, Sears has become the man on the spot in the Reagan campaign. A growing and vocal number of Reagan supporters question his credentials, his style and his operation of the campaign. Many consider Sears the archietect of Reagan's Iowa defeat.

"I don't know whether Ron could survive losing the New Hampshire primary, but I'm certain Sears couldn't," says one long-time Reagan fund-raiser in California.

Such staunch Reagan supporters as Iowa Lt. Gov. Terry E. Branstad, Manchester Union-Leader publisher William Loeb and Delaware Rep. Thomas B. Evans Jr. have told both Reagan and reporters that Sears has been too overprotective of the candidate and indifferent to some of the obstacles he faces. David Keene, a Sears protege who is now political director of the George Bush campaign, tends to agree.

"John has very little respect for candidates," Keene said. "I sometimes think that George Bush and those of us who work for him have far more respect for Ronald Reagan than Sears does. That debate [in Iowa, which Reagan ducked] was tailor made for Reagan."

In some respects, Sears is in the same spot as any other campaign manager whose candidate suffers an unexpected defeat. But in Sears' case, the objections run deeper.

Ever since he joined the Reagan campaign in 1976, the pragmatic process-oriented Sears has been a target of conservatives, especially in the South and West, where the attraction to Reagan is largely ideological.

He has been criticized for focusing too much on the Northeast, for under-rating the campaign skills of his candidate, for spending too much time with members of the press. And by those who have battled Sears and lost, he has been criticized as being authoritarian and power-hungry.

Hugh Gregg, who ran Reagan's New Hampshire campaign in 1976, and is now performing the same role for Bush, considers Sears "a ruthless, manipulative man who wants total control."

Few candidates could be indifferent to such comments, and Reagan is not. Without publicly criticizing his campaign manager, Reagan has taken the lead in abandoning the front-running strategy that was supposed to carry him into the White House.

If this change in tactics is galling to Sears, he does not show it. He was impersonal and professional in analyzing Reagan's prospects for a group of Washington reporters Monday, conveying no sense that he is under pressure.

When a reporter jokingly asked Sears the week before the Iowa caucuses whether Reagan "appreciated his greatness," Sears smiled and replied, "It remains to be seen whether I'm great or a fool."

It is unlikely that even Sears' most antagonistic critics would consider him a fool. Except among a handful of extreme conservatives, Sears is widely praised for engineering Reagan's designation of Richard S. Schweiker as his prospective running mate in 1976, a maneuver that kept Reagan's chances alive when they were dying. This time, Sears proved accurate in his assessment that Reagan could maintain his popularity in the polls during 1979, even when he was not a declared candidate.

But once Reagan became an announced candidate, the front-running strategy did not work well. Reagan's refusal to debate in Iowa and his limited campaign time there played into the strategy of opponents who were eager to suggest that Reagan is too old or out of touch to become president.

And Sears also has been criticized, as he has been in the past, for deficiencies in Reagan's organization.

"John's strategy is fine," said one long-time Reagan supporter familiar with the mechanics of campaigns. "But his operational ability isn't up to his strategic capability. In 1976, he didn't bother even to screen the television commercials before they went on the air. He is a thinker, rather than an organizer," this supporter said.

"Sears proves that genius is no substitute for hard work," said Reagan ex-aide Lyn Nofziger, who was driven from the campaign by Sears in August.

Sears' track record would indicate that he can survive such criticism. He has been coming back from adversity all his life, which began 39 years ago on a farm near Syracuse. His father died in a fire when he was 10. He went to Notre Dame where he led supporters of John F. Kennedy at a mock convention, and became a Washington lawyer. He was a Northeast strategist for Nixon in 1968 and the troubleshooter who was supposed to keep Spiro T. Agnew out of trouble in the general election campaign.

When Reagan failed to win the early primary as expected in 1976, some of his followers made Sears the target.

What is clear is that Sears marches to a different drummer than others in the campaign and that he is as upset with the political process as others are with ideology.

"Politics is not a science," Sears says. "There are not truly correct answers except in the results. The best part of politics is the human interplay and the testing of what's possible against the ideal and seeing what progress you can make toward that. The whole nature of the process is you should run the best race you can, that's the best thing you can do. What you've really got to guard against is a creeping feeling that what you are engaged in is ordinary. Because it really isn't."