While nearly overshadowed by eruption of the Kennedy rumor mill, Gov. Edmund G. Brown Jr. showed last weekend he can prove a major disruptive figure in the New Hampshire Democratic primary.

During nine hours of campaigning, Brown was consistently upstaged by hints that Sen. Edward M. Kennedy is emerging as an active candidate. But one of Kennedy's shrewdest supporters here, ex-state chairman Joanne Symons, looked hard at Jerry Brown and came up with this conclusion: "I was impressed. He is a very, very formidable candidate. I was prepared to see a flake, but he is not."

Neither Symons nor anybody else gives Brown or President Carter a chance against Kennedy as an avowed candidate in New Hampshire. But two glimpses of Brown at Democratic picnics were enough to convince Symons that 1) he could actually defeat Carter in a two-man race if Kennedy clearly withdraws and 2) he could prove real trouble to the Draft-Kennedy movement if their effort here remains merely a write-in.

Brown's favorable outlook in a two-man race with Carter derives from the president's deplorable political condition here. Although the Democratic establishment from Gov. Hugh Gallen on down still backs him, there were no Carter buttons to be seen at the two picnics. The Carter table at the Hillsborough County picnic in Nashua was ignored, despite the attraction of a color propaganda film supplied by the Democratic National Committee.

The president's political deterioration was underlined at the Nashua picnic when Sen. John Durkin, facing a tough reelection battle, surprised Gallen by virtually endorsing Kennedy and dumping Carter. But what if Kennedy finally bows out? Durkin suggested the president is so weak Brown might defeat him if he managed 40 New Hampshire visits to retail votes. But, added the senator, Brown is a California wholesale politician unsuited for cozy politicking.

Durkin is wrong. Brown is excellent in small groups, and showed it in three private sessions with private citizens wedged between the public picnics. Although he will not come here 40 times, he does plan 30 visits (providing he can solve the problem of Republican Lt. Gov. Mike Curb, whose menace back in Sacramento forced Brown to halve this first major visit to New Hampshire).

Dressed in white shirt and double-breasted blue blazer, Brown appeared as a soft-spoken, thoughtful non-flake. He balanced his anti-nuclear and anti-defense appeals to the left (heartily applauded by the party activisits here) with balanced budget and business investment appeals to the right (not applauded at all).

Brown blundered briefly by calling for federally-aided reforestation in a state that considers itself too woodsy for its own good. A more serious error was bringing along a busload of young Brown rooters (mostly residents of Massachusetts) who shoved real New Hampshire voters out of the way to get into camera range. This Hollywood transplant was rejected by the Granite State.

Brown also appeared a shade peculiar at a Democratic picnic in Dover when, in contending candidates don't discuss the real issues, he declared: "Holistic medicine (that is, medicine for the whole body) -- you certainly don't hear much about that in campaigns." A young matron wearing a Draft-Kennedy button nudged her husband and whispered sarcastically: "You certainly don't."

She and other Kennedy enthusiasts in the crowds certainly did not change campaign buttons after seeing Brown. But almost all such voters we questioned told us that in a Carter-Brown choice, they would pick Brown. That buttresses Joanne Symons' view of a two-man race.

Whether she is equally correct in assessing Carter's strength against a Kennedy write-in is dubious. And, of course, if Kennedy is an avowed candidate, even Brown's own advisers privately admit he stands no chance.

Brown did fire some oblique shots at Kennedy, warning against "rerunning the hollow rhetoric of another era" and complaining that "a lot of people want to fight the battles of yesteryear." Gov. Gallen, steadfast for Carter, hit Kennedy from another angle. He contends Kennedy is not ideologically compatible with "right-of-center" New Hampshire Democrats and notes tha; Kennedy leaders Symons and state Executive Counselor Dudley Dudley were associated with losing McCarthy-McGovern-Udall left-of-center campaigns.

But in truth, the Kennedy phenomenon here as elsewhere defies ideology. In the Draft-Kennedy movement Symons and Dudley are collaborating with conservatives who have been blood enemies in the past. Kennedy's popularity is such that Jerry Brown, impressive candidate though he is, is reduced, like all other Democrats, to Waiting for Teddy.