Tom Bradley's long lead in his race to become the first elected black governor in U.S. history has narrowed significantly in the final days of the most expensive, and one of the most negative, California gubernatorial campaign in history.

In three weeks, Republican Attorney General George Deukmejian has cut Bradley's lead from 14 to 3 percentage points, according to tracking polls. He has scheduled a blitz of weekend television commercials in an attempt to close the gap by Tuesday, Election Day.

The commercials, similar to last-minute attacks that helped Deukmejian upset Lt. Gov. Mike Curb in the GOP primary, deride what has been Bradley's strongest selling point: his nine years as the popular mayor of Los Angeles.

"My lead is very firm and I am very confident of victory," Bradley said of the Oct. 24 California Poll by Mervyn Field that showed him still ahead, 47 to 41 percent.

Bradley aides said they anticipated a last-minute Deukmejian assault and have begun to air commercials defending Bradley's record. The Bradley ads insist that he strengthened the city police force rather than weakened it as claimed by Deukmejian.

Bradley, 64, and Deukmejian, 54, have always been bland and cautious campaigners. In this race they have characteristically said almost nothing about the most difficult issue expected to face the new governor: a multibillion-dollar deficit in the state budget.

But both have unleashed an unusual amount of venom in the barrage of television and radio commercials which should push total spending in this race over the $13 million mark; so far Deukmejian has raised $6.4 million and Bradley $6.3 million.

This week, Deukmejian launched one of his most effective attacks, in the candidates' last debate in San Diego Tuesday and in a last-minute television advertisement.

Since Bradley became mayor nine years ago, Deukmejian said, "Murder has gone up 80 percent, robberies have gone up 105 percent. And his answer to the crime problem in Los Angeles was to cut the size of the Los Angeles police department by 600 sworn peace officers."

A spokesman for the Los Angeles Police Department said he did not want to become involved in a debate between the candidates but said the department had 426 fewer sworn officers now than it had in 1973 and experienced a drop of 331 in the number of officers assigned to field duty from the end of 1973 to the end of 1981.

Told of the police figures, a Bradley spokesman said the mayor had replaced officers doing desk work with civilians in order to free them for street duty. Despite what the police spokesman said about a decline in street officers, the Bradley spokesman said enough money was not budgeted to pay 365 more street officers than existed in 1973.

Despite Deukmejian's emphasis on stronger police forces and tougher judges, polls have shown the economy replacing crime as the issue of most concern to California voters.

Bradley points to 2,000 new jobs created in Los Angeles during his tenure while Deukmejian says he will remove the "anti-business" atmosphere in Sacramento allegedly generated by Gov. Edmund G. (Jerry) Brown Jr., the Democratic Senate candidate.

Bradley remains, however, a conservative Democrat who won considerable support from Republican businessmen here. He has passed several opportunities to attack the economic policies of President Reagan.

Neither gubernatorial candidate has directly raised the race issue, although Bradley often begins his speeches with remarks about the "California dream" and the effect the election of a sharecropper's son such as himself might have on other poor children.

The issue flared anyway two weeks ago, when veteran Republican strategist Bill Roberts, then Deukmejian's campaign manager, said a hidden anti-black vote would put the Republican over the top if he trailed in the polls by 5 percentage points or less.

"It's just a fact of life," Roberts told reporters. "If people are going to vote that way, they are certainly not going to announce it to a survey taker."

Bradley called the remark an "insult." Deukmejian said race was "not a relevant subject" in the campaign, but the furor led to Roberts' resignation.

Some polls indicate that the number of voters favoring Bradley because of his race, including many blacks, greatly outnumber those who oppose him because of it. The California Poll reported 5 percent would not be inclined to vote for a black, but 12 percent would not be inclined to vote for someone of Armenian descent, as Deukmejian is.

Deukmejian apparently has had some success in sullying Bradley's image by establishing a vague connection to the shortcomings of the outgoing Democratic administration in Sacramento. It has had enough impact that Bradley repeats in almost every confrontation with Deukmejian this sentence: "You're running against me, not Jerry Brown."