Two years ago, Gov. Edmund G. (Jerry) Brown Jr. stood before a crowd of 20,000 at Cuesta College near here, enthusiastically joining the chant "No on Diablo Canyon! No on Diablo Canyon!"

Today, as demonstrators gathered to try to blockade that same nuclear power plant in an action partly inspired by Brown's public stand, the governor is nowhere to be found. To the surprise of some protesters and local politicians, he has announced the state's full support to defeat the blockade.

It is the latest in a series of shifts Brown has made as political tides have turned, but added to his other rapid turnabouts on the Mediterranean fruit fly, state tax limits and other issues, many California politicians wonder how many more reappraisals Brown's career can stand.

If the slow-starting blockade attempt forces the use of hundreds of police and drains the county treasury, as many predict, Brown will have erected one more barrier in the way of his planned move to the U.S. Senate.

"As in the case with the Medfly, the governor's past positions have immensely increased both the dangers and expense involved to the state," said Rep. Paul N. (Pete) McCloskey Jr. (R-Calif.), also a candidate for Sen. S.I. Hayakawa's GOP seat.

Brown and his staff have been actively resisting efforts by the Pacific Gas and Electric Co. to acquire a federal license to operate Diablo Canyon, which could provide 20 percent of northern California's electricity.

Brown has insisted that security and emergency preparedness plans are insufficient and that a major earthquake on a nearby fault could damage the plant and cause radiation leakage.

Brown has intervened with the Nuclear Regulatory Commission to stop the license, encouraging antinuclear groups to focus their efforts on the Diablo Canyon plant as the most likely target among several plants under construction nationwide.

With the governor so actively opposing the plant, "We figured if we couldn't shut it down here, where could we?" said Mary Moore, one of the spokesmen for the antinuclear coalition Abalone Alliance.

Brown said this week that he will continue to fight the plant before the commission, which must decide within a week whether to allow fuel loading and low power testing at Diablo Canyon, and could also take the case to the courts.

But he did not back off from his earlier statement that the demonstrators' rights to free speech and assembly "do not extend to blocking roads or infringing public and private property rights."

He designated Highway Patrol Commissioner Glen Craig to help local police officials with the protest, and promised more aid under California law if the number of demonstrators climbs into the thousands. With about 400 having checked in since the Abalone Alliance sent out its appeal Wednesday, local officials are beginning to doubt that they will need much state assistance.

Jerry Diefenderfer, a Republican on the San Luis Obispo County Board of Supervisors, said Brown's offer of support indicated to him that "the law-and-order issue has become the focus of the 1982 election." He noted the local disgruntlement at projections of a cost of up to $250,000 for police, prison and court expenses if the numbers of demonstrators arrested moved into the thousands.

Also, voters statewide appear to support the opening of Diablo Canyon. The most recent survey by Mervin Field's California poll in October showed 56 percent in favor of allowing the plant to operate, with 31 percent opposed and 13 percent saying they didn't know.

Demonstrators pitching their tents in a large field full of dried straw outside of town today appeared cynical about the motives of the nation's most antinuclear governor, who would probably prefer that the protest stay small and that the TV and newspaper reporters gathered here fade away.

"The guy is a politician," said Lance Potter, 28, an English instructor at the University of Southern California, who is here to protest. "Here it is, and his actions speak louder than his words."