RICHMOND, SEPT. 18 -- Republican legislators called today for a retroactive reduction in the state's income tax, saying the state is "wallowing in a multimillion-dollar surplus."

State Sen. Wiley F. Mitchell Jr. (R-Alexandria) said it is "unconscionable" for Democratic Gov. Gerald L. Baliles and the Democratic-controlled General Assembly to retain the current rate structure, saying it will produce a $300 million-a-year windfall, largely from middle-income residents.

Republican legislators will introduce a bill when the assembly convenes in January that would roll back the top tax rate, effective from last Jan. 1, from 5.75 percent to 5.5 percent, Mitchell said at a news conference at the Statehouse. Their plan would further reduce the rate to 5.25 percent beginning Jan. 1, 1988, he said. The reductions would apply to incomes above $14,000.

A family with a taxable income of $30,000 would save $40 this year and $75 next year, according to figures supplied by the GOP. At $40,000, the saving would be $65 this year and $125 next year.

Baliles dismissed the idea as "the latest of many proposals to deal with the budget surplus. All these proposals will be reviewed as the budget takes shape between now and January."

State Republican Chairman Donald W. Huffman said the matter will be an issue in the legislative election Nov. 3.

"It's going to be part of every local race," said Mitchell, who is chairman of the joint Republican legislative caucus and is unopposed for a fourth term.

"I'll be interested to see if the {Republicans} maintain a united front for this plan after the November elections," said state Democratic Party Chairman Lawrence H. Framme III.

The GOP plan would not affect the $154 million surplus Baliles announced two weeks ago. Mitchell said that money "is already in the bank."

"We want to sound the alarm," said Mitchell, who said most taxpayers won't be aware of the increase until they file returns next spring.

"Democrats are misrepresenting the effects" of legislation passed earlier this year that was designed to return any windfall to taxpayers, Mitchell said. "It returns only about one-half," he said, and if the amount of the windfall was underestimated, as Mitchell said he believes it was, "it returns less than half."

Mitchell said the legislature "promised to avoid an income tax increase" when it raised the sales tax at a special session on transportation last fall and "reinforced that promise" during the regular session this year by setting up a transition fund to return any windfall revenue.

"But when we asked the General Assembly to fulfill that promise by cutting the tax rates, we were told there was not enough money," Mitchell said. Now, he said, there is enough money.

He said the legislation passed earlier this year took care of the state's poorest citizens by removing 250,000 of them from any income tax obligation and made other low-income taxpayers the major beneficiaries of refunds from the windfall that was collected largely from middle-income wage-earners.