The governor of California, an incongruous three-piece suit in a sea of jeans, was lounging on the Capitol steps and explaining, between chomps on an apple, what he called "some basic truths of energy politics."

"In the dialectical way of American politics," Gov. Edmund G. Brown Jr. began, "decontrol of oil will ..."

"Governor," someone interrupted, "the cameras are over here."

"No, wait-this is good," Brown demurred. "Let me finish this thought. Let's see-in the dialectical way of American politics, decontrol of oil will, or I mean may, lead to further government control of the oil companies.

"Brown's audience-an assortment of reporters,, politicans, antinuclear leaders and miscellaneous activists who had flocked to Brown's side as soon as he appeared at yesterday's rally against nuclear power-looked up quizzically. But before he could explain what he had in mind, he was off to be interviewed by a self styled "public interest video cooperative" that wanted to film the antinculear movement's hottest political property.

One day after President Carter's political swing through California, Jerry Brown had brought his own unannounced but obvious presidential campaign to the seat of the federal government. By the end of yesterday's rally it seemed clear that Brown got the best of the exchange.

The President's trip to California was marked by modest crowds, lukewarm applause and repeated expressions of public dissatisfaction with the Carter administration's energy policies.

The governor's expedition to Washington, in contrast, was a command performance-the rally organizers worked hard to get Brown to come before a crowd estimated at 65,000 to 75,000 that roared its approval of Brown's policy of opposition to nuclear energy.

The roars were louder before Brown addressed the crowd than after, because the governor's speech, a grab bag of leftist rhetoric that Brown shouted from notes he scrawled on a small card barely five minutes before he was introduced, did not go over well.

There were, moreover, small pockets of anti-Brown sentiment here and there in the largely liberal crowd, because some groups still have not forgiven him for embracing the drive to balance the federal budget.

For most of those who blanketed the Capitol grounds yesterday, though, neither a weak speech nor a willingness to support some conservative causes was enough to tarnish the image of a politican who has never wavered in his opposition to nuclear power.

On that issue, at least, Brown was their man, and if, as speaker after speaker predicted yesterday, the antinuclear wave achieves the force that the antiwar movement did 10 years ago, Jerry Brown will be riding its political crest.

Brown was aware of all that yesterday, and as he prowled the plaza at the west front of the Capitol, waiting for his turn to address the crowd, he smiled like the cat that had swallowed the canary.

Unlike most politicians, Brown generally leaves his staff at home when he travels. This weekend, he said, he flew to Washington with only one aide, traveling on the same plane as two other stars of yesterday's rally, Jane Fonda and her husband, Tom Hayden.

Brown walked to the Capitol alone yesterday away, but he was not alone long. As soon as he was spotted, he picked up followers like a magnet passing through a pile of paper clips. He spent most of the day surrounded by a buzzing swarm of cameras and microphones.

"Does Three Mile Island mean that nuclear plants have had it? someone asked, and Brown nodded his head. "Let's say-they're in the process of having had it," he said."That's a more precise way to put it."

Brown said it would not take another major nuclear accident to strenthen the antinuclear driv.

"The next assault on nuclear power," he said, "will arise from the evacuation plans, when people start to take a look at these emergency evacuation plans.

"Those plans-they're a bad joke. The zone of the LNZ-Or what is it? What is that thing-The LPZ, that's it, the Low Population Zone, the zone where you have to move people out if there's a crisis-those zones are not realistic."

All this talk was interrupted by a constant flow of people, obscure and famous, who came around just to share the presence of a man who might be president.

Fonda and Hayden, who have not always agreed with Brown in the past, were generous in their praise of the governor's stand against nuclear power. "We're not endorsing anybody for president yet," Fonda said, "But it looks like Kennedy's not going to run, and ..."

Brown also ran into Bella Abzug, the feminist leader, who berated him for failing to appear at last summer's pro-ERA rally in Washington. "Yeah, well, I'm a complex person," Brown said, and before Abzug could think of a comeback, he was off to shake another hand.

"You know, this movement, its amazing," Brown said. "Its a strange blend of left and right, both sides are-"

But he was interrupted again, this time by a young man wearing a serape, a green Mexican-style hat, blue porcelain earrings and hair down to his waist.

"That one-I think he's on the left," Brown said. CAPTION: Picture 1, With the Capitol in the background, a portion of the crowd estimated at 65,000 settles in by the west steps to hear speeches opposing the United States dependence on nuclear nower. By Frank Johnston-The Washington Post; Picture 2, Reminiscent of the antiwar gatherings of the '60s, this crowd gathered at the Capitol yesterday to protest U.S. dependence on nuclear energy. Capitol police estimated the throng at 65,000 to 75,000. By Larry Morris-The Washington Post; Picture 3, California Gov. Jerry Boown, Actress Jane Fonda and her husband Tom Hayden await turn to speak. By Frank Johnston-The Washington Post