Registered nurses at Prince William County Hospital yesterday voted for the second time in two months to be represented by a union, marking the first successful attempt to organize health care workers in Northern Virginia.

By a vote of 63 to 44, a narrower margin than in the first election, the nurses at the Manassas hospital decided to form Prince William Health Care Professionals, Local 724, of the Service Employees International Union.

Hospital administrators said they will decide in five days whether to contest the vote.Yesterday's election was held because the hospital objected to what it termed unfair labor practices allegedly used by the nurses at the June 19 election.

Dave Cromer, assistant director of organization for the service employes, said he doubts the hospital will fight this election because "it would look embarrassing."

Union organizers had claimed that the hospital's objection to the first election was a stalling tactic to allow more time to win nurses over to their side.

E.L. Derring, spokesman for the hospital, said, "For the past month, we have made every effort to inform our nurses of the pitfalls of unionization," adding that the hospital will bargain in good faith if the National Labor Relations Board certifies the election.

The 128 nurses have already formed an 11-member bargaining unit and have drafted demands, including premiums for weekend work and hospital-paid leaves for continuing education.