With less than a month remaining before California's June 8 primary, major contests are becoming sharply defined:
* Barring a major upset, either US Rep. Barry Goldwater Jr. or San Diego Mayor Pete Wilson will be nominated by Republicans to face Democratic Gov. Edmund G. (Jerry) Brown Jr. in the November contest for US senator.
* Mayor Tom Bradley of Los Angeles is coasting to the Democratic nomination for governor, but a fresh California Poll shows Lt. Gov. Mike Curb and state Attorney General George Deukmejian virtually even going to the wire in the Republican gubernatorial primary.
* Another just-published California Poll shows a surprising turnaround in voter sentiment on the Peripheral Canal -- the ''big ditch'' proposed by Governor Brown and the Legislature to speed water from the Sacramento River system to the arid south. In a March voter sampling, 46 percent favored the canal, 39 percent were against it, and 15 percent undecided. A sampling taken in the first week of May shows 35 percent for, 45 against, and 20 undecided.
Mervin D. Field, director of the nonpartisan polling organization, attributed the reversal to weakening support for the canal in southern California, which supposedly would benefit most from the project, and ''ever--enlarging, overwhelming rejection of the canal by northern Californians.''
Responses to California Poll questions indicate, said Mr. Field, that ''concerns about costs of the canal are by far the most important reason for the increase in anticanal sentiment in the south.''
The poll's findings in the two-man GOP contest for the gubernatorial nomination have been particularly interesting. In January, Mr. Deukmejian led Mr. Curb by 43 to 32 percent, with 25 percent undecided; a March voter sample showed that Curb had managed a stunning reversal, leading 50 to 40, with 10 percent undecided. But a poll result issued May 12 has Curb barely ahead. He has 47 percent, Deukmejian 45, and 6 percent are undecided.
Meanwhile, Mayor Bradley has been busy campaigning in his low-key manner, aiming not so much at the June 8 primary as at -- November and the Republicans.
In the crowded contest for the Republican senatorial nomination, Field surveys dating back to early last fall have shown Barry Goldwater Jr. in front, with Pete Wilson second and US Rep. Paul N. (Pete) McCloskey third. But while the Goldwater total has shrunk, Mr. Wilson has edged closer to the leader.
The most recent California Poll on the GOP Senate race, released March 31, showed that Mr. Goldwater had dropped five points, to 29, while Wilson had gained six, to 24. Mr. McCloskey hit the 18-point mark last October and hasn't budged since.
If the trend continues, Wilson could well pick up enough of the 11 or 12 percent in the ''undecided'' category to edge out Goldwater on June 8. The San Diego mayor has been a much more aggressive campaigner than his chief rival, arguing that he is the strongest candidate the Republicans can pit against Jerry Brown.
Meanwhile, Governor Brown has been spending more time tending to state business than on the campaign trail, apparently realizing that a high percentage of Californians have a negative view of his record as state chief executive. While that factor will not deny him the nomination, it could be decisive in November, where he may be the underdog.
Trailing far behind in the Democratic senatorial primary, but leading a quartet of relative unknowns, is author Gore Vidal. Poorly organized and financed, the Vidal campaign has produced some interesting newspaper copy and some stinging jibes at the governor -- but is not expected to furnish an upset. Pollster Field has found a lot of anti-Brown sentiment in the state, among Democrats as well as others. But this is seen as a much more important factor in November than in June.
The rise in opposition to the canal could be an indication that the forces aligned against it have mounted a more effective media campaign. Both sides have spent much money over the last several weeks for television advertisements that are high in emotional impact but low in substantive arguments.
Television and money are major factors in other contests, too. Candidates have been amassing as much as they can for the ''stretch run'' ads that are being seen more and more frequently on television.
The last official report by the California Fair Political Practices Commission showed that by mid-March those running for the legislature and statewide offices had collected $24.8 million and that gubernatorial candidates had spent $8.4 million, a record for a primary in the state.
No figures on Senate candidates are available yet from the Federal Election Commission, but the San Francisco Examiner has reported that Jerry Brown has $2. 3 million in his campaign fund. He is holding down primary spending while trying to build a $5 million general election war chest. This will counteract the $5 million to $8 million some sources say the national GOP will pour into an attempt to keep him out of the US Senate.