A bipartisan group of Prince William County residents is circulating a petition to have the chairman of the Board of Supervisors elected at-large by all citizens of the county rather than by other board members.

The move would change the county form of government from county executive to urban county executive, the form used by Fairfax County.

The petition calls for a referendum this November. Under Fairfax's system, the chairman of the Board of Supervisors is elected at-large, but under the current Prince William system the chairman is elected from among the seven board members, said petition drive leader Claude V. Bradshaw, a substitute teacher and former candidate for the House of Delegates.

Bradshaw said last week the Citizen's Coalition for Good Government needs 10 percent of Prince William's 42,000 registered voters to sign the petition before the referendum can be put on the ballot this November. He said he did not know the number of signatures collected so far but predicted the coalition would have 4,600, or more than 10 percent, by Sept. 1.

"People are very enthusiastic about it," he said.

All seven members of the county board, who represent their respective districts for four-year terms, are up for election this November. Bradshaw said that, if approved, the change in county government would take place in 1987, the next scheduled election of the board.

County Attorney John Foote said last week, however, that if the referendum is passed, the county would have to change its form of government in a year. That would mean the board elected this November would serve only one year, then face another election in November 1984. Foote said the board elected in 1984, with its chairman elected at-large, would serve the remaining three years of the term.

"I think the idea is worth looking at, but the timing seems all wrong," board chairman Kathleen K. Seefeldt said. "We would have three elections in a four-year period. The new board would not even have time to set policies when it starts in January if the members had to run again that November."

Bradshaw said a Prince William chairman elected at-large, as is Fairfax's chairman John F. Herrity, would be responsible to all the people in the county, not just those from his district.

He also said the new form of government would enable residents to vote for two supervisors--the supervisor from their respective district and the chairman. He said a popularly elected chairman also may have more clout in state government.

"The state legislators would know this person represents a large block of voters," Bradshaw said.

A drive in 1972 to change to the Fairfax form of government was halted in Prince William when it was discovered that a state law prohibits a county from changing its form of government more than once in three years. The county switched to its present form in 1970.