U.S. Sen. Frank R. Lautenberg passed his first, and perhaps most significant, hurdle for a fifth term yesterday by holding back an aggressive challenge from U.S. Rep. Robert E. Andrews, a Camden County Democrat tired of waiting to fulfill his dreams of higher office.

Lautenberg must, of course, face former Republican U.S. Rep. Dick Zimmer in the fall. Though Zimmer promises a hearty campaign, he will be fighting history. New Jersey voters haven't liked a Republican for Senate since 1972.

Last night's early, unofficial returns bore out the much-denied but obvious north-south split. Andrews won in parts of the south, and Lautenberg swept the north.

Andrews polled strong in his own congressional district, which runs through Camden, Gloucester and Burlington Counties.

In recent races, the Camden County-based South Jersey Democratic machine, headed by George Norcross 3d, has been racking up victories as far away as Cape May County.

But the pattern barely held up yesterday. Andrews took a slim edge over Lautenberg in Cumberland and Cape May Counties that overlap a state Senate district now represented by a Democrat who had the machine's backing last year. He won by small margins in Atlantic and Salem Counties.

Lautenberg's forces pulled off 3-1 and 4-1 blowouts in densely Democratic Bergen, Essex and Hudson Counties.

Although Andrews took endorsements and help from some leaders in central and northern New Jersey counties, it wasn't enough to overcome a four-term U.S. senator.

Even though voters had told independent pollsters they were concerned about Lautenberg's age at 84, yesterday they declared they wouldn't abandon him for a 50-year-old congressman.

Andrews played on Lautenberg's age, even running an ad that said the senator would be 91 at the end of the term he now seeks.

The Andrews candidacy embodied the desires of other congressmen and women across the country.

"Show me an incumbent congressman - not only in New Jersey - and I'll show you a wannabe senator," said Mike Murphy, Andrews' general campaign chairman. "There aren't many people who serve in the Congress who don't fantasize or wish they could be in the upper house, if for no other reason than it's more prestigious, and they don't have to run every two years."

Beyond Lautenberg and Andrews, the people with the biggest stake in yesterday's low-turnout Democratic primary were "the other six [Democratic representatives] who are not named Andrews" but who want Lautenberg's seat, said Peter Woolley, director of Fairleigh Dickinson University PublicMind Polling Institute.

And the state party establishment wanted control over the seat.

In this campaign, Andrews went around the process, working the party leaders without the competition of other wannabes. At the top of the fight card it was just he and Lautenberg. Morristown Mayor Donald Cresitello ran a very distant third.

The state's other congressmen rallied around Lautenberg.

U.S. Rep. Frank Pallone, whose district runs through Monmouth and Middlesex Counties, cut radio ads featuring him and Lautenberg and imploring voters to "support the two Franks" on election day. When it looked like Middlesex County's top party leaders would defect to the Andrews team, Pallone fielded slates of candidates to run against the party's picks.

U.S. Rep. Steve Rothman, who represents parts of populous Bergen County, helped keep Bergen Democratic chairman Joe Ferriero on Lautenberg's side in the early days of the race. The chairman, it turned out, spent the final days of the primary being distracted by a federal subpoena of his business records and a leadership challenge in his county organization.

Republicans had a three-way race that came down to Zimmer, selected by a party establishment that wanted a moderate and known quantity to push against the tide in a strong blue state in the fall.

As the primary was closing, Zimmer announced he received support from U.S. Sen. John Ensign (R., Nev.) who heads the National Republican Senatorial Committee, and others. Democrats, signaling the race was on, said Zimmer was getting help from "Bush Republicans."

Before finding Zimmer, the party's brass got behind Anne Evans Estabrook, who dropped out in March after suffering a mild stroke, and then Andy Unanue, a Goya Foods heir, who also dropped out.

Running second in the GOP Senate primary was State Sen. Joseph Pennacchio, a Morris County dentist, who captured some early support but was apparently overwhelmed by party insiders who wanted their own candidate.

Murray Sabrin, a Ramapo College professor, got a single county endorsement in Gloucester County, but could not parlay it into a win there. Pennacchio came in first in Gloucester.