A sharply divided Virginia Association of Counties endorsed the idea today of a statewide bond issue for road building, a proposal advanced by Northern Virginia and other suburban interests but opposed by several key legislators.

Also, the association, an influential lobbying group in Richmond, approved by a narrow margin a Fairfax proposal that will allot votes within the organization to counties according to their populations. The change will give Fairfax County, by far the state's largest locality, more votes than the next largest four counties combined. In return, Fairfax will have to more than double the amount of money it pays to the association, from $27,000 to more than $60,000.

The key votes on both the road bond and the new voting system were 34 to 27, with the state's once-dominant rural counties arrayed against Northern Virginia and its suburban allies.

The association's positions are to be placed before the 1986 General Assembly in January. But the suburban-rural split in the group, particularly on transportation financing questions, could presage a struggle between the two coalitions within the state legislature itself, officials here agreed.

"It's a fair indication of what you'll see in Richmond," said Del. Robert Andrews, a Republican who represents Northern Fairfax and part of Loudoun counties.

The association endorsed a general road bond proposal that specified neither how large the bond should be nor how the borrowed funds would be distributed. Most officials here for the association convention seemed to agree any statewide bond would have to be at least $500 million. The state's road needs are estimated at $16 billion.

State Sen. Edward A. Willey, chairman of the Senate Finance Committee, has already declared his opposition to any road bond next year.

Virginia, with a strong pay-as-you-go tradition to finance road building from current funds, has never had a statewide road bond issue.

A coalition of the state's rural counties tried to defeat the road bond proposal, suggesting instead an increase in the state's gasoline tax and vehicle titling fees. The rural officials urged that the revenue from these sources then be distributed according to a 1977 formula that favored less populous areas. That formula was redrawn early this year in the state legislature to funnel more road money to Northern Virginia and other suburban areas.