It is a delicate game that President Carter is playing in his unannounced and unofficial campaign for reelection.

In Iowa Friday, Carter was telling a group of county officials about how Washington is infested with "snake oil salesmen," peddling inflation cures, and others whose heads are filled with pipe dreams about easy solutions to the nation's energy problems.

"You know," he said softly, "there are times when I wish that Washington was Iowa's 100th county."

There could hardly be a purer expression of the Washington "outsider" the use of White House Power, the president casuallye with its unspoken suggestion that more wisdom resides in the cornfields of th Midwest than in the bureaucratic citadels of the nation's capital.

But a few minutes later, in the same speech to the same audience, it was a different Jimmy Carter speaking, in this case the preeminent government insider dispensing largess from a bottomless federal goody bag. He was pleased to announce, the prsident told the local officials, a series of grants and loans to rural areas and several administrative actions to guarantee an adequate supply of oil, natural gas and diesel fuesl to farm communities.

"I will not allow rural America to run dry," he said.

So it goes, nine months before the first 1980 presidential primary, as Carter seeks to meld the outsider themes that worked so well for him three years ago with the powerful tools of incumbency.

In the process, Carter is relying increasingly on some of the techniques and themes of 1976 while he exercises the power he enjoys in 1979.

Last month in New Hampshire, in another deft example of the use of Hwite House power, the president casually announced that he had ordered a substantial increase in the nation's reserves of home heating fuels, a vital concern in energy-dependent New England.

And on Friday in Des Moines, in an example of the careful rhetorical shading his 1976 campaign was accused of, Carter answered a question about the future of nuclear power in the United States with an expression of "deep concern" for safety factors and an assertion that nuclear power should be used only as "a last resort" to meet the nation's energy needs. The answer possibly obscured, at least momentarily, the president's often-stated belief that the country cannot afford to turn away from nuclear power.

Today, Carter was in California, the back yard of an almost certain rival for the Democratic presidential nomination, Gov. Edmund G. (Jerry) Brown Jr., appealing to Mexican-Americans, a growing force in national Democratic politics.

The president stayed here overnight Friday at the home of a Mexican-American family, a method he has used to remain identified with ordinary Americans. But even before Carter arrived here late Friday night, White House officials made sure to release the text of a letter from the president to the nation's governors, asking for their cooperation and suggestions in dealing with the problem of illegal immigrants, the bulk of whom are from Mexico.

Any candidate can stay overnight in a private home, and many, like Carter in the early stages of his 1976 campaign, must resort to such devices to save campaign funds. But only a president can attract much attention with a letter to the nation's governors, particulary when he says it grew out of his personal conversations with the president of Mexico.

White House officials are well aware of the double-edged sword they hold in their hands. With Carter's popularity sagging in the polls, they are hoping to recapture some of the magic of 1976, when Carter the outsider surprised his party and the country to become the Democratic nomiee and eventually the president. When they see Carter at his campaign best-for example last month at a "town meeting" in New Hampshire-they are encouraged.

"I think the president's ability to regenerate good will out there has been underestimated," one official said.

At the same time, the White House is not neglecting the advantages of the insider. One of these is Carter's ability to barnstorm the country in an official capacity, doing himself maximum political good at minimum cost. So long as the president does not directly solicit votes or other support for 1980, most aspects of trips such as the one he completed today are considered "official" and paid for by the government, White House press secretary Jody Powell explained to reporters.

White House speechwriters have been briefed on the intricacies of federal election laws, lest they put into the president's mouth words that might turn an "official" trip into an expensive "political" excursion.

For the next several months, probably until the fall, when Carter is expected officially to announce that he is running for reelection, it will be much the same. "You bet," one White House official said, when asked if more such presidential travel can be expected. "You will see more groups coming in, more laying on of the hands."

Through all of this, the president is likely to decline, as he did in Des Moines Friday, any official reelection announcement.

"I think it is best for me, in a time of some excitement about progress and some concern about problems, to remain a fulltime president," he told a news conference. "It is too early for me to get involved in any discussion about an upcoming election."

From the news conference, Carter went to a gathering of the Democratic Party of Iowa, the state that gave him his first crucial boost in 1976. At the end of his brief talk to the Iowa Democrats, the president said:

"I don't like to approach Iowa people on opposite sides of a velvet rope. So I would like to ask you, if you don't mind, to come by one at a time and let me shake your hand and thank you personally and pledge my complete dedicated service to make you proud of our nation. And if you don't mind, I would like to get an individaul photograph with you as well. Okay?

It was all there in that concluding statement. There was the modest candidate, unwilling to be separated from his friends by even a velvet rope, quietly asking to photographed with each of them. He didn't say that each of them would likely prize a picture of himself or herself with the president of the United States.

In Washington, before the trip, a White House official put it another way. "In every way, shape and form," he said, "this administration is looking for friends." CAPTION: Picture 1, President Carter Begins a busy day in Los Angeles by jogging with his host, Stephen Roddriguez. AP; Picture 2, Carter bids farewell to Stephen Rodriguez family of East Los Angeles before going to Cinco de Mayo celebration.