President Reagan, a former governor of California, will make the defeat of current Gov. Edmund G. (Jerry) Brown Jr. his top priority in the 1982 Senate campaigns, according to White House political aide Edward Rollins.

Brown, who is almost certain to win the Democratic nomination to the Senate in next Tuesday's primary election, was described by Rollins as "the exact philosophical opposite of the president."

Rollins, a former staff member in the California legislature, is as much a product of California politics as are Reagan and Brown. He said that because the state is "the president's home base, it is important that California has a senator who can help the president carry some of the momentum in the Senate; . . . even if it wasn't Jerry Brown, it is important to hold that seat."

Republican politicians are assessing this year's Senate races with an eye on 1984, however. If the GOP doesn't pad its current 54-to-45 Senate majority (with one Independent), it could lose control in 1984, when 19 Republican seats and just 14 Democratic ones will be decided.

Brown has done little campaigning for his party's nomination, which he is expected to win easily. He has concentrated instead on raising money for the fall election, which will be as difficult as the primary will be easy.

Unlike the Democrats, the Republicans have waged a hotly contested Senate primary, with their leading candidates--San Diego Mayor Pete Wilson, Reps. Barry M. Goldwater Jr., Paul N. (Pete) McCloskey and Robert K. Dornan, and Maureen Reagan, the president's daughter--each claiming to be the one to "beat Jerry Brown."

Polls show that any of them would beat Brown, who has high negative ratings. Mervin Field of the California Poll says that "when you ask people what they don't like about Jerry Brown, you've got to stand back" under the deluge of complaints.

But Field speculates that "Jerry Brown might have an angel guiding him." Field, who believes that Reagan will have "the single largest effect" on the outcome of the November election, says that, without an upswing in the economy, Republicans who try to "wrap themselves in Reagan's cloak" may find it "a suffocating blanket."

In a speech last weekend, Brown said: "I don't care who they nominate, because they're all clones of a disastrous, unfair economic policy that will be massively repudiated in November."

Rollins, noting that Brown already "has 2 million bucks in the bank," predicted that each side will spend about $5 million on the general election.

"We will do everything we can to reinforce Brown's negatives," said Rollins. Brown, he said, "is not handicapped by conviction, he is willing to turn on any issue." The California Poll lists "wishy-washy" and "opportunistic" as two of his biggest negatives among voters.

Rollins said that Reagan "will spend a little time out there" campaigning against Brown, probably in August when the president is at his Santa Barbara ranch.

At a Republican fund-raising dinner in Los Angeles last week, Reagan promised to keep Brown in California, "where he belongs," and suggested Brown could "raise Medflies." Allusions to last summer's Mediterranean fruit fly crisis are popular among Brown's opponents.

But none of them gets as big a laugh as Brown did last week when he presented to a gathering of editorial cartoonists a mock proclamation that he read aloud, using a fly-swatter as a pointer.

"There is no question," said Rollins, "Brown is a smart politician."