For Democrats in California, there's no ducking the touchy issue of immigration

California is where Walter Mondale, Gary Hart, and Jesse Jackson come face to face with something relatively new on the American scene: Hispanic political power.

Experts say that on Tuesday, when California votes, Hispanics could be pivotal. As many as one-third of the convention delegates chosen here that day may have received strong Hispanic support.

All the Democratic candidates are keenly aware that there is one super-charged issue that can influence the votes of Hispanic voters: immigration laws.

The United States Census Bureau estimates that more than 1 million California residents are illegal aliens, mostly Hispanic. The actual number, according to federal officials, may be twice that high. There are believed to be as many illegal aliens in California as in the 49 other states combined.

Thousands of these illegal residents are relatives or friends of Hispanics who will be voting in the June 5 primary here. This makes the issue personal, emotional, and potentially explosive.

None of this has been lost on Walter Mondale, who has been the chief Democratic beneficiary of Hispanic votes this year.

Mr. Mondale has moved deftly in Florida, Texas, and now here in California to hang onto as many as 3 out of 4 Hispanic voters. As the campaign nears its finish, his position has become more and more in tune with the demands of the Hispanic community.

Specifically, that means Mondale has opposed the Simpson-Mazzoli immigration bill that is now working its way through Congress.

In early primaries, Mondale said, ''I'm not for the Simpson-Mazzoli bill in total.'' But by the time voters went to the polls in Texas (a state with an estimated 186,000 illegals and a large bloc of Hispanic voters), Mondale had toughened his stand. He said flatly: ''I'm against this bill because I think it's wrong.''

Mondale went further. He used his influence with House Speaker Thomas P. O'Neill Jr. to get the vote on the bill delayed until after the California primary.

The Rev. Jesse Jackson has also serenaded Hispanic voters, but without too much success so far. His recent trip to Mexico, however, could help him do better with Hispanics in California than he did in earlier contests.

Gary Hart has sided with Hispanic demands on key immigration issues. But the Hispanic community's lack of familiarity with Senator Hart has kept his vote low.

The strong influence of the Hispanic community on the three leading Democratic candidates is a vivid lesson in American politics.

Poll after poll shows that most American voters want quick action on the issue of illegal immigration. The issue, however, isn't really a hot one. People are quietly concerned about it. But in most cases, non-Hispanic Americans are not so worked up about the issue that it could change their votes.

Many Hispanic voters, however, consider the issue a litmus test for candidates. Mr. Mondale particularly had to get on the Hispanic side of the issue, or get badly hurt in his marathon race against Mr. Hart and Mr. Jackson.

In the long run, however, the issue could haunt the Democrats. While they have lined up solidly against Simpson-Mazzoli, President Reagan has supported it , and apparently most members of Congress do as well. In the fall, it could be a plus for the President.

Mondale explains his opposition to Simpson-Mazzoli this way: ''This (bill) will be a horror for the Hispanics who live in this country, and they deserve better from us. . . . We've never tried effective law enforcement at the border. . . . Our Border Patrol has been shortchanged. There aren't enough of them. They aren't getting the support they need.''

Hart and Jackson say the problem of illegal immigration is rooted in economics. ''We will not solve the immigration problem by Simpson-Mazzolli or any bill like it,'' says Hart. Jackson blames ''the crippling poverty and disease across the border.'' Tougher border enforcement is no answer, he says.

The bill that has drawn fire from these Democrats has several provisions that in effect strike a compromise among the various sides on this issue.

For those who want to get tough on immigration, the bill would make it a crime for any employer to knowingly hire an illegal immigrant. This provision is designed to do away with the economic incentive for anyone to cross the border illegally. If an employer were found to have illegal aliens working in his business, he would be required to have all future job applicants show some kind of identification before being hired. Such ID would have to consist of a US passport, or a US birth certificate, or a social security card along with a driver's license. Certain other ID cards, such as those issued by a state, would also be acceptable.

This provision has been criticized as a step toward a national ID card for all Americans - a criticism that is hotly disputed by proponents of the bill.

While discouraging new immigration, the bill would make things easier for many illegals already here. It would grant permanent resident status (though not citizenship) to illegal aliens living in the United States since Jan. 1, 1982.