unning against Millicent Fenwick for the Senate is "frustrating as hell," said Democrat Frank R. Lautenberg. "I'm fighting a legend."

Lautenberg is frustrated by what he calls the "media hype": the fact, for example, that the 72-year-old congresswoman, a former Vogue model, is celebrated in the Doonesbury cartoon strip as Lacey Davenport.

"She's all style, no substance," he complains. "You don't hear about the Fenwick bill. You don't hear about accomplishments. You hear about Fenwick the cartoon character, Fenwick the pipesmoker, Fenwick the model."

But partly because of a withering negative television and radio ad campaign, Lautenberg has cut Fenwick's once imposing 18-point lead to 5 points, according to Republican tracking polls.

Lautenberg, 58, stepped up his attacks in the last two weeks of the campaign, calling Fenwick "eccentric," a reference to an offhand remark by former president Gerald R. Ford about the congresswoman's pipesmoking.

All this could change, however, as Fenwick fights back with her own ad campaign.

Fenwick, who calls Lautenberg "my dear opponent," nonetheless is infuriated by his ads, which charge her with voting against a $122-a-month minimum Social Security benefit and in favor of student loan cuts.

"I didn't remove the minimum benefit," Fenwick scolded Lautenberg in a recent debate "That is such a lie. How can you be so awfully naughty?"

With her throaty Katharine Hepburn voice, her expensive jewelry and elegant tweed suits, the feisty blueblood has yet to lose an election since her ascent to the Bernardsville Board of Education in 1938. She served in the state assembly and was elected to Congress from her well-to-do northern district in 1974.

Fenwick is better known in Washington for her outspokenness than for her legislative craftsmanship. However, as a politician, a label she deplores, she is an indefatigable performer, racing from one end of the state to the other in a battered gray Chevrolet, undaunted by the pacemaker implanted in her chest, and quoting Thucydides, Mary Queen of Scots, Racine and King Solomon.

In the southern New Jersey pinelands, she spent a recent morning gushing over the cranberry harvest: "You have to be a genius to farm," she told one landowner, as three television crews followed her around the bogs. Later, she regaled a local crowd by recalling that she'd been known as "Outhouse Millie" after her bill in the legislature required portable johns for field workers.

That evening Fenwick drove from a Princeton fund-raiser to the Morris County NAACP dinner. "She was a member of my board in the '50s, when it wasn't popular," said Rev. T.R. Goyens, former president of the NAACP chapter. "I'm the social action commissioner for 105 A.M.E. churches. She has all of our support."

Fenwick says the main campaign issue, in a state with 9.2 percent unemployment, is "jobs, jobs, jobs. It's agony. When I get a letter from a woman whose husband has lost his job, I want to scream." Lautenberg agrees that jobs are the issue.

"She opposed the two largest job-producing projects in New Jersey history, the Meadowlands and casino development," says one Lautenberg ad, referring to her assembly votes against a sports complex and legalized gambling.

The ad concludes, "What could be on Millicent Fenwick's mind in Washington?" Her supporters call that an attempt to smear her as a silly old lady. "Character assassination," as Sen. Nicholas F. Brady (R-N.J.) put it.

Fenwick's ads strike back, calling Lautenberg "a Jimmy Carter Democrat who favors more spending and opposes the balanced budget amendment."

For all the fury on the airwaves, Lautenberg in person is a charming man with a wry sense of humor that rarely comes across in his rhetoric-choked speeches. Chairman of Automatic Data Processing, which he helped build from a five-man operation to a multi-billion dollar business, he says his record of creating 16,000 jobs there gives him "damn good credentials for this Senate job. I would have to say darn near perfect."I

Son of an immigrant mill worker, Lautenberg also cites his philanthropic activity: former chairman of the United Jewish Appeal and founder of a cancer center in Israel.

He spent $1.4 million of his own money to beat six contenders in the primary, including Hudson County boss Joseph A. Le Fante, who, he says, won't support him now because he has refused to pay off Le Fante's campaign debt. He says he will spend another $1.5 million of his own in the general election.

Fenwick has refused to take political action money, and will probably be outspent by the Democrat 2 to 1, despite putting $200,000 of her own into the race.

On a walking tour down the main street of Jersey City, a working-class city, Lautenberg was preceeded by a sound truck that proclaimed, "Get Even With Reagan. Vote with Frank Lautenberg."

Fenwick, who campaigned with Reagan last month, is carefully distancing herself. "Yes, we must stay the course," she said recently. "But with a change in direction."