The Prince William Board of County Supervisors is expected to tap Terrence Spellane to serve as chairman for 1991, a year when budget crunches, building projects and political maneuvering will dominate the agenda.

Spellane, who served as vice chairman last year, was elected to the board in 1987 as a Democratically endorsed independent representing Coles. As an employee of the U.S. Customs Service, Spellane is prohibited by the Hatch Act from running for office as a member of a political party.

"I'm looking forward to it," Spellane said. "This'll be a tough year, but I think the board will pull through."

He will be the last person to hold the rotating chairmanship because residents voted to make it an at-large elected post, which will be filled in November.

All seven supervisors also will be up for reelection, and they are expected to either eliminate or add one magisterial district to prevent the possibility of tie votes.

To further complicate matters, both the local districts and those of the state senators and delegates must be redrawn this year to reflect the 1990 Census. Prince William is expected to gain a delegate's seat and part of a state Senate seat.

Several supervisors -- 1990 chairman Robert L. Cole (D-Gainesville), 1989 chairman Edwin C. King (D-Dumfries) and 1988 chairman Kathleen K. Seefeldt (D-Occoquan) among them -- have said they are considering a run for either the at-large chairmanship or a seat in the state legislature.

While preparing for the elections, the Board of Supervisors will have to cope with an unusually difficult budget year. State cuts of more than 5 percent have combined with slumping tax revenue to make money especially tight.

"The continuing economic difficulties of the county and the 1992 budget are tops on my personal list," said County Executive James H. Mullen, who said he hopes to avoid layoffs.

Several supervisors also have said they hope to reduce the real estate tax rate from its current level of $1.36 per $100 of assessed value.

"It's quite a challenge to keep a balance between our resources and our needs," Seefeldt said.

While looking for ways to reduce the tax burden on an already weak economy, the board also must find money to staff and equip the new Chinn Regional Library and Recreation Center, scheduled to open this fall.

Transportation improvements also will take shape around the county. Parking lots and stations must be built for Northern Virginia's first commuter rail, scheduled to open in October, and construction will continue on the Prince William Parkway, linking Interstate 95 with mid-county, and on Davis Ford Road.

"We've got to look at making sure we have a way to get people from the community to the trains, and not just parking lots," Spellane said.

Mullen said he also hopes to make progress on plans for a Route 234 bypass that would run west of Manassas, linking Dumfries Road and Interstate 66.

The supervisors also will look for ways to streamline human service programs when they consider a citizens committee report that advocates combining social services, community services and several other programs under one board.

Affordable housing also may become a hot topic this spring when the supervisors consider a measure to allow developers to build more densely if they agree to set aside some of the homes for lower-income people who otherwise couldn't afford to buy or rent homes.

County officials said they are looking to the state legislature to grant them the same taxing and borrowing powers that the cities of Manassas and Manassas Park already have.

Several unpopular issues, most notably jail expansion and waste disposal, are unresolved from last year. County officials said it is unlikely that much progress will be made, because building a jail or a trash burner would be very expensive, and neither one is needed immediately.