There is a peculiar, paradoxical quality to the current stage of Ronald Reagan's presidential campaign. The more he tightens his grip on the Republican nomination, the more concerned he and his advisers seem to be about his ability to hold on to the personal credibility he will need as a contender for the White House this fall.

The Reagan bandwagon was rolling this week, collecting delegates and endorsements at a pace that promised to foreclose any threat of an upset from challenger George Bush.

But aboard the fast-moving Reagan jet, no one seemed to have time to enjoy the victories he was winning. There was just too much anxiety that any time the candidate opened his mouth, he might put his foot in it.

When Reagan began this tour on Tuesday in Indianapolis, he was coming off a rough fortnight of television, newspaper and magazine stories questioning the accuracy of the facts he has cited to back up his energy, economic and welfare policy views -- and the claims he has made about his accomplishments in eight years as governor of California.

Although Reagan complained that the adverse publicity was the result of "journalistic incest," he compounded the problem by an acknowledged blunder in claiming that Vietnam war veterans were being denied the benefits that earlier American fighting men had enjoyed.

The unwritten commandment for this week was: No more goofs! Ed Gray, recently recruited from his job as a savings and loan executive to resume the press secretary role he played in Reagan's Sacramento days, told reporters Tuesday night that various steps had been taken to tighten the operation. "Watch us," he advised.

By every normal measure, the four-day swing -- which ended here Friday with a rally at the Alamo that had a fired-up Fiesta crowd cheering Reagan's fervently patriotic oration -- was a smash hit. In auditoriums and hotel ballrooms from Philadelphia to Bismarck and from Terre Haute to Houston, enthusiastic crowds reveled in Reagan's well-rehearsed applause lines. Meantime, his operatives were producing endorsements from leaders in New York, New Jersey, Delaware, Ohio and other states where Bush had hoped to rally a last-ditch drive if he can defeat Reagan in Tuesday's Pennsylvania presidential popularity contest.

The "psychological inevitability" of Reagan's nomination was being strengthened every day.

But aboard Reagan's chartered aircraft, a different struggle was being waged -- the struggle to restore credibility to his statements and, more broadly,to dispel any notion that Reagan is an intellectual lightweight who would be over his head in the White House.

Reagan publicly professed no great concern about the threat of a credibility gap. "I haven't wavered or changed my position at all," he told a Philadelplhia press conference. "I know we go through these things from time to time, and right now there have been some articles and programs questioning some of my statementss . . . I will stand on, and continue to use, the figures I have used, because I believe they are correct. Now, I'm not going to deny that you don't now and then slip up on something; no one bats a thousand . . . But I have faith in my sources, and I'm not vacillitating." a

Despite Reagan's professed unconcern, it became clear this week that steps are being taken to answer the challenge to his credibility and to see that, in the future, he will be better briefed. A "white paper" is being prepared on his record as governor, rebutting a Los Angeles Times story questioning some of his own claims on the subject -- a story which aids said irritated Reagan more than any other published criticism.

Gray has ordered a news service printer installed at Reagan's Los Angeles headquarters so the traveling party can be apprised of late developments during stopovers on the campaign trail. Until now, he said, headquarters staff members have listened casually to an all-news radio station and relayed items they might hear, leaving Reagan on the road often unaware of news events about which reporters wanted to question him.

The number of interviews Reagan gives to individual reporters aboard the plane has been cut back, Gray said, in order to allow him more time for readings and briefings. At almost every stop, Reagan opened his press conference with a carefully researched statement that made him sound like an expert on a particular topic.

Veterans of the Reagan tour noted what they thought were significant omissions of some standard speech lines whose accuracy has been challenged.

Moreover, Reagan seemed to go out of his way to display concern for accuracy. He made a point of telling a Philadelphia Republican dinner, "I have been saying we won seven straight special (congressional) elections, but that is not correct. There have been 10 electons, and we have won seven of the 10."

But even with all the precautions, there were slipups. In attacking the administration's "cheap food policy," Reagan asserted that President Carter had "thrown open the floodgates" to inexpensive beef imports -- a substantial misstatement.

And the problem is potentially significant, because it raises a basic question about a man who is now viewed seriously as a possible president.

The problem was exemplified when a local reporter in North Dakota noted that Reagan has explained his mistake on the Vietnam veteran's benefits by saying he had been given the information by two retired four-star officers and, as an ex-captain, "two bars don't question four stars." The reporter asked whether he would be equally uncritical as president in accepting the judgment of the Joint Chiefs of Staff.

No, said Reagan: "I am seeking advice from people of contrary views, and sorting it out myself. And I think the record of eight years in California of doing the same thing indicates that I do not make hasty decisions on major issues on just hearing a remark."

Convincing the voters of that is now a high priority in the Reagan campaign.