President Carter's oil price and tax policies won backing here today from the board of electors of the National Association of Counties, the nation's only lobbying and policymaking group for county governments.

The 100-member board voiced unanimous support for the administration's decision to decontrol domestic oil price, and for its proposed "windfall profits" tax on increased oil company earnings generated by decontrol.

The vote came on the eve of President Carter's scheduled keynote address here before an estimated 4,000 county officials attending the 44th annual NACO convention.

NACO, a $2-million-a-year operation, represents 1,800 of the 3,104 county governments in the United States. Because it is a largely rural and Republican organziation, today's vote was seen here as significant.

The United States Conference of Mayors, a largely big city Democratic group, approved similar oil price and tax resolutions last month at its annual conference in Pittsburgh.

The differences in the resolutions approved by the two local government groups are:

The mayors called for a "transitional" windfall profits tax - one that would be removed after an unspecified period of time.

The county leaders sided with the administration in calling for a permanent windfall profits tax.

The mayors would use most of the windfall profits tax revenue to help the poor meet fuel and energy conservation needs. For example, approximately 50 percent of the tax revenue would go to the poor, 25 percent would go to energy research and development, and the remaining 25 percent would be set aside for mass transportation development under the mayors' proposal.

The county leaders did not specify a percentage distribution for the windfall profits tax revenue. But many board members said today they favored using a large share of the money than would be used by the mayors for energy research and development and mass transportation.

The full NACO membership is expected to approve the board action on Tuesday.

"The resolutions already have passed the convention's environment and energy and transportation committees, and today they were unanimously approved by the board. I see no opposition developing against the proposals on the convention floor," said Harvey Ruvin, commissioner of Dade County, Fla., who also serves as NACO's energy policy chairman.

Following a heated debate Saturday, featuring representatives of the Nuclear Regulatory Commission and pro- and anti-nuclear groups, the NACO board took no action today on the issue of nuclear power plants. Instead, the board called for stricter rules on the disposal of nuclear waste and exploration of alternative energy sources, primarily wind and solar.

On another major question today, the board sided with members of Congress who want ot severely restrict the use of federal tax-exempt revenue bonds issued by state and local governments to help finance single-family home mortgages at below market rates.

In a surprise, the board approved a resolution that would prohibit the use of such bonds to finance single-family homes and private, multiple-family developments. Those who favor the use of the bonds said that the board's action, if left standing and if passed by Congress, would make it difficult for low-and moderate-income families to buy homes. They have vowed to fight the resolution on the convention floor.