Fairfax County officials say the county could lose $1.9 million to $2.4 million in federal revenue sharing funds -- the money the federal government divides among local governments -- next year if the Carter administration's proposed changes for the program are approved by Congress.

Under the plan Congress is now considering, a new funding formula for the revenue share would be instituted, resulting in more money for poorer jurisdictions and less for rich ones such as Fairfax.

The county currently receives $6.6 million annually under the program. County officials say their policy in the past has been to use the federal dollars for two non-recurring expenses that benefit all citizens: improvement of secondary roads and payment of debts. Several officials say because of the conservative use of the money in the past, a loss of funds would hurt the county but would not be catastrophic.

The reduction in revenue sharing dollars is not a major funding problem by itself -- the money amounts to less than 2 percent of the proposed county budget of $446.5 million. But budget analysts for the county say a revenue sharing cut, combined with a potential loss of $10 million in federal impact aid to schools, adds up to financial trouble.

"The revenue sharing money is just a straw -- but whether it may be the straw that breaks the county's back remains to be seen," said one county official. "Add that money to the impact aid we may lose and you get a nasty problem."

The impact aid loss plus revenue sharing cuts could equal 3.6 percent of the county's proposed budget.