For one wisp of a moment, at a Labor Day press conference under a cloudy Florida sky, George Bush almost broke his self-imposed rule and let the thoughts in his head come tumbling out of his mouth.

"Do you find yourself a little more cautious about the things you say now?" a local reporter asked the No. 2 man on the Republican national ticket.

"Yeah," laughed Bush, "a lot more. It's different being. . ." The laugh left his voice and he fell into an uncharacteristic stutter ". . . being in ah . . . you know . . . in ah . . . in this role," he concluded lamely, letting all the nouns and adjectives that might have said more remain unspoken.

These days, Bush is all too conscious that the Republican presidential campaign does not need any more mistakes -- "glitches," he calls them -- to divert the nation's attention from the planned attack on President Carter's economic and foreign policies. So Bush doesn't want his words to be too different from those of presidential candidate Ronald Reagan. Or too loud. Or too spontaneous.

In the traveling echo chamber that is the GOP vice presidential campaign, Bush wants no unnecessary noise.

If this means that he and his statements are relegated to relative obscurity, appearing on page 5 or 25 of newspapers around the country, so be it. As a Republican presidential candidate last winter and spring, he generated weekly front-page headlines. But now, "Let's face it," said press secretary Peter Teeley, "the only way we're going to make national headlines is if we screw up. And we don't want to do that."

"The difference is that Ronald Reagan is the nominee for president of the United States," Bush said in an interview on his way to the Texas GOP convention here this weekend. "Which means to me is that what counts are his views, his strong convictions, his approaches, his suggestions.

"What I should be doing is articulating those as best I can and avoid getting him into trouble by overstating or contradicting. He has enormous burdens. And I don't need to add to it by going for the flamboyance of the year award."

A minute later, he put it more succinctly: "It's not my neck. It's not my fortune," Bush said. "He [Reagan] is the head of this ticket." In practice, this supporting role highlights Bush's strength as a politician. He is good at the broad-brush attack, drawing cheers in cities from Orlando to San Antonio with his denunciations of the United States' lack of military preparedness, and the economic woes that he says were caused by the overregulation and overspending of the Carter administration.

When Reagan makes a mistake, as he did last week with his remarks about the Ku Klux Klan, it is Bush's role to smoothe over the troubled waters.

This, however, is not always easy. Communication between the Bush and Reagans camps while not bad, particularly with former Bush campaign director James Baker in a high-level spot in Reagan's operation, is not always instantaneous.

For nearly 24 hours after Reagan made his KKK gaffe, a testy Bush had to face a barrage of press questions at one Florida airport after another. Reagan had gone to ground in Detroit, as he and his advisers drafted and redrafted explanations of the remark, but Bush was exposed to the world and he didn't know quite what to say about it.

It was not until an advance man ran up the steps of Bush's plane after i landed in Miami late Tuesday afternoon that the vice presidential candidate heard of Reagan's explanation and began to talk of the controversy with ease.

The affair left a residue of discontent with some Bush staff members, one of whom told a friend, "I wish they could get back to planning the campaign instead of having to backtrack and cover themselves all the time."

But even in this awkward situation, Bush returned to the offensive in short order, praising Reagan's "forthright apology" at every stop and lacing into Carter for his failure to apologize about such slip-ups as his reference to "Montezuma's revenge" at a 1979 luncheon in Mexico City with President Jose Lopex Portillo.

By week's end, Bush was getting as much applause with the Montezuma's revenge line as he was earlier with his litany about the woes of the working man under a Democratic administration.

Bush's role, however, is not entirely rhetorical. Part of his job is to raise money for local campaigns and state party organizations. This is more important now than in 1976, since the federal election laws have been losened slightly in the interim.

Now, local and state party organizations may pay for such things as bumper stickers, brochures and phone banks. These expenses, under the new law, will not be charged against the $29 million that federal law allows the Reagan-Bush campaign to spend. So, in Miami, in Pittsburgh, in Florence, S.C., and at a half-dozen other stops, Bush did his duty as the guest of honor at one fund-raiser after another, chatting with and encouraging potential donors.

The net take at these events was in excess of $150,000, according to campaign aides.

Throughout the seven-state swing that began Monday, Bush was talking almost exclusively to the converted: His audiences were largely white, well-heeled middle- and upper-class Republicans. The most notable exception was a Cuban breakfast in Miami.

The one event that might have been less predictable, a trip to a senior citizens activity center in the Little Havana section of Miami, was pulled off his schedule. Aides said there was too much to do that day, although workers at the center were told that both scheduling and security reasons prompted the cancellation.

In these relatively safe settings, there is little risk that anything could go wrong. There is little risk of surprise and little risk of mistakes, unless Bush makes them himself.

"This new role doesn't mean that I can't express myself," the candidate said. "But why make the needless mistake anyway?"