In Tuesday's New Jersey primary, with most attention on the Republican senatorial race between Rep. Millicent Fenwick and former Reagan speechwriter Jeff Bell, the Democratic candidates for the seat vacated by the resignation of Pete Williams can be forgiven if they feel a trifle neglected. Democratic pollster Peter Hart summed it up for his candidate, millionaire businessman Frank Lautenberg. "Frank," he said, "you're a pygmy among pygmies."

Whatever their size, there are a lot of them. "The party is in a state of flux. Anybody could win it," says Rep. James J. Florio, who lost the governor's race by fewer than 2,000 votes last November and was the Democrats' leading prospect until he declined.

The leaders among the nine hopefuls who didn't decline are Lautenberg and former representatives Andrew Maguire and Joseph A. LeFante. Among the others are Mercer County Freeholder Barbara Boggs Sigmund, daughter of Rep. Lindy Boggs (D-La.), attorney Howard Rosen, former banking commissioner Angelo R. Bianchi and Passaic County Freeholder Cyril Yannarelli.

Maguire, a liberal, is supported by liberal unions such as the machinists and ladies' garment workers, environmentalists, feminists and the Bergen County party. LeFante is backed by other party regulars, who think Maguire is too liberal and self-righteous. Lautenberg, who built a data processing firm into the world's largest computing company, has spent $1 million of his own money on television advertising.

All nine Democrats have one common campaign theme, however. All are running against President Reagan's economic policies.