A Republican congressman from California has written a letter to 10,000 southern Fairfax County voters charging that there is a "county income tax scheme" being planned by "bug-spending liberals" in the Virginia House of Delegates that must be stopped by electing a conservative whom he supports.

"Here's how the county income tax scheme will work," freshman Rep. Robert K. Dornan wrote in his letter urging a vote for Lawrence D. Pratt, a House of Delegates candidate from southern Fairfax County. "After you've figured out your Virginia income tax you'll be forced to add on another 10 to 20 per cent when the state will parcel out to the counties."

Fairfax officials and legislators interviewed yesterday said there is no such county income tax planned.

"That's not true." Fairfax County Supervisor Joseph Alexander said of the letter which he received last Friday. "The letter is pretty much indicative of the Nixon dirty trick tactics." he said.

"There's nobody trying to increase county taxest," said three-term Republican Del. James H. Dillard II, who was defeated for re-election in a GOP primary last June.

"Nobody is even planning" a county income tax, It's a straw man."

But Pratt, who prepared the Dorman letter, disagreed. He said a bill that would have placed a county surtax on the state income tax was introduced in the surtax on the state income tax was introduced in the 1976 legislature by Del. Carrington Williams (D-Fairfax) and 17 cosponsors. Consideration of the bill was delayed until this year's General Assembly session when Williams said he saw that the bill had no chance of enactment and asked that it be killed by a House committee.

Williams said the bill was designed to make taxes more equitable by increasing the amount taxpayers would pay on their income, while requiring a corresponding decrease in the amount of local real estate taxes. He said such a shift in taxes would aid power people on relatively stable incomes.

However Pratt said, "I'm very much unimpressed with those trade-offs. I doubt when the bill would have actually come out there would have been any reduction in property taxes."

Although the bill was killed in committee and Williams is not running for re-election. Pratt said "the political dynamics of the legislature" are such that the bill probably will be introduced again in the next session in January.

The Dorman letter was discussed at yesterday's county Board of Supervisors meeting. "I want to know who these liberals are that he Dornan is talking about," Alexander said.

Dornan's letter begins: "As I write you this letter, big spending liberals in the VIrginia House of Delegayes are planning a county income tax bill that will hit the pocketbooks of every single Virginia tax-payer."

Alexander replied. "The (county) Board of Supervisors would have to request something like that." He said he knows of no such request.