The weekend after she was escorted down the steps of Air Force One arm-in-arm with President Reagan, Rep. Millicent Fenwick (R-N.J.) was back on the road again in her dented Chevrolet.

She was speaking to the annual Cumberland County Republican brunch in this rural New Jersey hamlet, where tomatoes are "4 lbs./$1" and unemployment is 15 percent. She talked of the frustrations of being a minority member in the House, of failed urban renewal programs in Camden, of deficit spending and, finally, of the president.

"I thought it was awfully nice of him to come," she said. "That man cares about human beings. He doesn't . . . derive his interest from statistical charts and demographic calculations, but from the problems of people. And it shows."

She went on with reverence about the president, but said not a word about his economic policies.

That is the approach Fenwick and other Republican candidates apparently consider the best way to get mileage out of their association with Reagan in this campaign. They want to be part of Reagan's nice-guy image, bask in his role as national and international leader, and have as little as possible to do with Reaganomics.

This comes as no surprise to the White House, where the prevailing view is that Reagan must also be careful this fall not to squander his time working for candidates that are beyond hope, or in areas experiencing a depression.

"We're not going into a state that could hurt the president, like Wisconsin and Michigan," said one administration official. But he added that some Senate races are considered urgent for Reagan if Republicans are to keep control of that chamber during the second half of his term.THE NEW JERSEY SENATE RACE

The result is that, so far, Reagan is playing an entirely different role in congressional campaigns this fall than he did two years ago as a candidate. Then he was free to work full-throttle, and held out his coattails to all riders.

This year, he is performing more limited tasks: fund-raising and lending a hand to promising GOP Senate candidates like Fenwick "who have the ability to win" and where Reagan's presence might "make a difference," the administration official said.

How much of a difference this presence--and the limits on Reagan's campaigning this fall--are evident in the New Jersey Senate race for a seat that White House officials expect to win for the GOP. It was vacated earlier this year by Democrat Harrison A. Williams Jr., who faced Senate expulsion on Abscam charges.

Made famous by cartoonist Garry Trudeau as the model of Lacey Davenport in the comic strip Doonesbury, Fenwick, 72, wears her independence as a badge of pride.

While Fenwick enjoys a sizable lead in polls, she was desperate for fund-raising help from Reagan because of her refusal to accept corporate contributions.

Her opponent, Democrat Frank Lautenberg, 58, president of Automatic Data Processing, plans to spend $2.5 million this fall. Reagan's visit netted her $100,000 and several days of valuable media attention, but campaign strategists don't attach any weight to a presidential visit beyond that.

One White House official said that while a Reagan appearance in a sparsely populated state like Montana, where he campaigned last month, is a "major, major event," a day trip to New Jersey, the most densely populated state, results in only a "blip on the screen" in nearby Philadelphia and New York television reports. And Fenwick frustrated even that by closing her fund-raiser with Reagan to the cameras.

Another factor is that, like many other races, the New Jersey Senate contest is much more than a referendum on Reaganomics. Fenwick's personality and record are the chief targets of her rival.

"The president's policies will be an issue, but Millicent Fenwick is the larger issue," said Lautenberg's campaign manager, Tim Ridley.

Another element is what polls describe as the voters' "patience factor" with Reagan, making some Democrats fearful that confronting the president directly could backfire.

Still, at the Cumberland County GOP brunch, it was also clear that Republicans aren't clamoring to be associated with Reaganomics.

John J. Mahoney, a conservative high school English instructor running for the House, didn't mention the president in his speech. He had to cut it in half at the last minute, he said, so he left out the part about Reagan.

And state Sen. Jim Hurley said of the president's standing among GOP candidates: "He's not the Pied Piper."