In a pair of precedent-breaking primaries, Democratic voters yesterday chose former San Francisco mayor Dianne Feinstein to contest for the California governorship and black liberal Harvey Gantt to battle Sen. Jesse Helms (R) for a North Carolina Senate seat.

Feinstein bested state Attorney General John Van de Kamp and will face Sen. Pete Wilson (R-Calif.), the man the GOP drafted to hold the governor's office that incumbent George Deukmejian (R) is leaving after eight years. Feinstein is the first woman to win a major-party nomination for governor in California. {See story, Page A8.}

Their battle has been called the number one contest of the year, but Gantt, the first black ever nominated for the Senate in North Carolina, and Helms, who has defeated three white liberals in the past, also will mobilize support across the nation for their contest of ideological opposites.

In Iowa, abortion-rights supporter Don Avenson won the Democratic gubernatorial primary over state Attorney General Tom Miller, an opponent of abortion who tried to play down that issue. In November, Avenson will face Gov. Terry E. Branstad (R), who opposes abortion.

Nominations for eight Senate seats and five governorships were determined in voting that also spanned Alabama, Mississippi, Montana, New Jersey, New Mexico and South Dakota. No incumbents faced serious threats in the primaries.

In North Carolina, Gantt defeated prosecutor Mike Easley by 57 to 43 percent in a runoff primary that Easley forced when Gantt came up just short of the required 40 percent in the six-way primary four weeks ago. A poll last week showed Gantt in a virtual dead heat with Helms. Their general election contest will pit the Senate's most vehement critic of liberalism against a challenger who supports abortion rights, opposes the death penalty and favors unrestricted federal aid to the arts.

"It's a new day in North Carolina," Gantt told supporters, as returns showed he had apparently pulled in significant numbers of white votes, especially in urban counties. The turnout was especially large in Charlotte, where Gantt was mayor until two years ago.

Democrats celebrated their unity after a relatively harmonious primary, but acknowledged that Helms will pose a far tougher challenge for Gantt in November. Easley had complained in the final days of the runoff campaign that Helms was tilting the Democratic race by focusing radio ads against him, a charge the senator's aides denied. The loser immediately pledged his support to Gantt.

Abortion dominated the Iowa Democratic gubernatorial primary, where Avenson, the speaker of the House, had 40 percent to Miller's 32 percent, with banker John Chrystal, also an abortion-rights supporter, at 26 percent. Avenson had the support of the National Abortion Rights Action League, which broadcast commercials and mailed campaign literature on his behalf.

The Senate race in Iowa is expected to be one of the most competitive in the country this year and Sen. Tom Harkin (D-Iowa) and Rep. Thomas J. Tauke (R-Iowa) began campaigning against each other long before their formal nominations in yesterday's primary.

In other states:

ALABAMA. First-term Gov. Guy Hunt (R) will learn the identity of his Democratic challenger only after a runoff later this month. With three-fourths of the vote counted, Paul Hubbert, an official of the Alabama Education Association, led with 32 percent of the vote. State Attorney General Don Siegelman held a narrow margin over former governor Fob James for the second spot in the Democratic runoff, with Rep. Ronnie Flippo apparently falling short. The top two candidates will meet in a runoff June 26.

Hunt, Alabama's first GOP governor since Reconstruction, easily defeated two minor primary candidates in his run for reelection.

Sen. Howell Heflin (D-Ala.) won nomination to a third term against token opposition. Heflin's November challenger, state Sen. Bill Cabaniss, was unopposed for the GOP nomination.

A constitutional amendment making English the state's official language carried overwhelmingly.

MISSISSIPPI. The only Democrat who filed to challenge Sen. Thad Cochran (R) announced on the eve of the primary he was withdrawing from the race, leaving the senator unopposed in November.

MONTANA. Republicans, hoping for a replay this fall of the 1988 Senate election in which the underdog, Conrad Burns, defeated Sen. John Melcher (D), selected Lt. Gov. Allen Kolstad to oppose Sen. Max Baucus (D), seeking his third term.

Kolstad, the candidate recruited by the national GOP, defeated Bruce Vorhauer, inventor of the contraceptive sponge, by 43 to 36 percent with nearly all the vote counted.

NEW JERSEY. Sen. Bill Bradley (D) crushed an unknown primary opponent and is strongly favored in November to defeat Republican nominee Christine Todd Whitman, the former head of the state Board of Public Utilities.

The retirement of Rep. Jim Courter (R), who lost the governor's race last November, produced a spirited GOP primary. State Sen. Richard Zimmer defeated former New York Giants football player Phil McConkey and Assemblyman Rodney Frelinghuysen. Zimmer will face wealthy Democrat Marguerite Chandler in what has been a GOP stronghold.

NEW MEXICO. Former governor Bruce King (D), seeking a third non-consecutive term, defeated three primary opponents and will face former state representative Frank Bond (R) in November. King had 52 percent, with former attorney general Paul Bardacke second with 80 percent of the vote counted. Bond had 55 percent in the GOP primary, with state Senate Minority Leader Les Houston closest of the three trailing candidates.

Gov. Garrey Carruthers (R) is ineligible to succeed himself. Sen. Pete V. Domenici (R-N.M.) will be opposed by state Sen. Tom Benavides (D), who was also unopposed in the primary.

SOUTH DAKOTA. The November ballot is set for South Dakota's gubernatorial and Senate races. Gov. George S. Mickelson (R) will face former Democratic state senator and state party chairman Bob Samuelson. Sen. Larry Pressler (R) will face Democrat Ted Muenster, a veteran of state politics.

Staff researcher Bruce Brown contributed to this report.