He is as wide open as a Western range studded with sagebrush, this California cowboy who yearns to be a US senator. One of the reasons he wants to trade up from representative to senator, says Barry Goldwater Jr., is that he'd be 43 times as powerful as he is now as the Republican member from the 20th district. This son of a famous forthright father, Arizona Sen. Barry Goldwater, is as candid, as open about power as he is about chocolate pudding. He loves them both.

But does he enjoy power? he is asked. Goldwater tilts back in his black leather executive chair and grins. ''Oh, sure. Oh, I love power. I think power is fantastic. The power to walk into a room and influence direction, pick up the phone and get things done, for people or for yourself or whatever. . . . I always enjoy being on top of the totem pole. If you're not No. 1, you might as well be last. Power is an aphrodisiac. It can certainly give you feelings, perhaps false feelings, of superiority. That's kinda heady.''

Goldwater the younger tends to shoot from the hip when asked a question like that - a question other members of Congress usually approach as cautiously as a rattlesnake under a rock. The power brokers of Capitol Hill would have you believe, at least on the record with reporters, that they don't so much enjoy power as endure it.

But here is Barry Goldwater Jr. with the same refreshing outspokenness that has marked his father's career, talking about what a kick power is. Then he pauses and political altruism comes galloping in:

''But I don't look at power necessarily in those . . . although I enjoy those aspects of it, I look at it and enjoy power from the standpoint of effecting change. I spend 'X' amount of time trying to do my job, which is basically to make change where change is needed, whether it be getting a disaster program changed or advocating repeal of something. . . . The more power I have, the more effective I am in those hours that I spend doing what I'm trying to do. It's much better to effect change than to try and not be able to do it. And so I enjoy power from that standpoint, because I think it tends to complement the job and the responsibility.''

Young Goldwater never would have made it in the court of the Borgias. Innuendo and nuance are not his style. If he's read Machiavelli, it didn't take. As a friend of his says: With Barry, what you see is what you get.

At one point in a two-part interview, a minor question pops up. Earlier, I had been told by a reporter for another newspaper that Goldwater had been seen riding a motorcycle through the halls of the Rayburn Building. Is there any truth in that allegation? He laughs. ''A motorcycle? No, no. Not a motorcycle. That's how things get distorted. What he probably saw me do is ride a skateboard through the halls of Congress. Or roller skates.''

You do both? ''Oh, yes. The marble halls are fantastic for gliding around. I do it for kicks, keep my hand in, generally after everybody's gone home. Not a motorcycle, though. Not even a bike.'' He has set the record straight, with the air of a man who believes that if you think skateboarding or skating through the Rayburn Building is flaky, that's your problem. Now motorcycles, that's something different.

You look for boots the first time you see him. He is unmistakably Western: lean, tan, and as weathered as a wrangler, with a deep twangy voice that drawls out the words the way John Wayne used to. Like Wayne kicking the ashes of campfire at dawn, telling the other cowboys to hurry and saddle up ''cause we're burnin' daylight.'' It's no surprise that one of Goldwater's favorite authors is Zane Grey. He dresses in the Eastern uniform (shirt, tie, vested business suit), but the gray tweed jacket is slung over a table and the trousers are cut cowboy-style with squared-off hip pockets. No boots: brown moccasins. His light brown hair is cut as conservatively as his politics. He is backlit in the fall sunlight, so that his eyes look hazel. ''No, not hazel - blue,'' he corrects. ''Sparkling blue,'' he adds with a wry smile.

Well, sometimes not quite so sparkling. In our first interview he had taken the notorious red-eye flight from Los Angeles at midnight, arriving in the capital at dawn, groggy with jet lag, to begin a 12-hour Washington day after a weekend with constituents and campaigning. Nearly a year before the senatorial election, he's not burning any daylight; he's out there rustling up votes. Waiting to ambush him are several senatorial candidates who have already declared, including the incumbent Republican Sen. S. I. Hayakawa, as well as Democratic Gov. Jerry Brown and family friend Maureen Reagan, daughter of the President.

At this point Goldwater doesn't feel he has any close competition: ''I continue to lead in the polls despite the fact that several people have announced.'' But he acknowledges that ''Senator Sam,'' as he calls him, has an advantage as the incumbent and is also unpredictable. He also says, ''I never underestimate any potential candidate, including Maureen Reagan, because I know her well and know her to be capable on her feet as a campaigner.''

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