Gov. Edmund G. (Jerry) Brown Jr. today launched his candidacy for the 1980 Democratic presidential nomination by creating a fund-raising "exploratory committee" headed by his top campaign aide.

Tom Quinn, who will resign Tuesday as a member of the Brown cabinet and chairman of the California Air Resources Board to head the committee, said he anticipated that the two-term governor will make a formal announcement of the candidacy this fall.

"All of us who are in this, including Jerry, expected to be in it at least until the [Democratic national] convention next year -- and we hope beyond that," Quinn said.

Brown himself was in Mexico today, on a visit to President Jose Lopez Portillo that symbolized the governor's independent stance on a key foreign policy issue.

Ever since then-Energy Secretary James Schlesinger refused last year to pay the price asked by the Mexican government for natural gas, Brown has beem cpmtemdomg that the Carter administration is insensitive to the need for developing hemispheric economic cooperation. Brown's view is that the United States, is the natural customer for Mexican gas and oil and should try to forge a common market involving the United States, Mexico and Canada.

In New Hampshire, which will hold the first 1980 presidential primary on Feb. 26, Brown's entry was welcomed by a committee of five Democratic state legislators, all but one under 30 year old. Spokesman Jim Kaklamanos of Nashua, a 23-year-old law student and second-term member of the state House, said he flew to California last week at his own expense to meet Brown and urge him to campaign in New Hampshire.

"I like his unconventionality and the fact that he stands up to the establishment, Kaklamanos said. He said he respected President Carter but predicated "the demise of the Democratic Party as we know it" if he is renominated.

The treasurer of Brown's "exploratory committee" will be Jodie Krajewski, who will resign her post as director of administration in the governor's office. She is the wife of Jan Krajewski, a South Africa citizen and major Brown fund-raiser who arranged the governor's trip to Africa with singer Linda Ronstadt earlier this year.

Quinn said his committee hoped to raise the maximum allowed under the federal champaign law and qualify for matching funds. CAPTION: Picture, CHARLES W DUNCAN "I report directly to president"