Arlington County prosecutors yesterday opened their case against Dennis Sobin, the flamboyant Washington publisher and frequent political candidate, on a charge of conspiracy to pander. The prosecution called several employes of Sobin's Washington escort service to testify.

Sobin's lawyer, James Lowe, countered that the prosecutors are harassing Sobin, publisher of the newspaper Free Spirit, for having "the audacity to write about corruption in Arlington County, and in the Arlington County vice squad."

Sobin's trial in Arlington Circuit Court is the latest confrontation between Sobin and Arlington Commonwealth's Attorney Henry Hudson. Hudson has declined comment on Sobin's allegations that he is being harassed, and Sobin has never provided data to support his claim. If convicted, Sobin, who has pleaded innocent, faces up to 10 years in prison.

Hudson has taken a hard line against prostitution during the past two years. During that period, Sobin has written numerous critical articles about Hudson, referring to him as the "the demented d.a."

Last year, Sobin offered memberships in an adult swingers club he founded to anyone who would make a $100 donation to the political campaign of Brendan K. Feeley, who was running against the incumbent Hudson. Feeley disavowed the plan.

Employes of Mais Oui, Sobin's Washington escort service, testified in detail yesterday about the operation. They said they operated out of homes in Arlington and described engaging in various sexual acts for pay.

One former Mais Oui escort, Victoria C. Cooper, 28, testified that from 1982 to January of this year she had sex with up to 14 customers a week referred to her by the company, and that the great majority paid by credit card. Cooper said she did not deal with Sobin but with a man at Mais Oui who identified himself only as "Tom."

Assistant Commonwealth's Attorney Liam O'Grady said in court that "Tom" is one of 15 aliases Sobin used, and that he used at least 15 companies as part of an "outcall house of prostitution."

Gregory B. Simpkins, 30, testified that he started working for Free Spirit in April 1983, but quickly learned that Sobin was running an escort service out of the same office on H Street SW in the District. "I was expected to make these calls" to arrange liaisons, Simpkins said. "He assured me this is all legal, this is not prostitution."