WERE IT MERELY a case of legislative bomb-throwing to wake up members at a wee-hour session of the House of Delegates, Virginians could toss it off lightly -- but all of a sudden in the middle of the night after a marathon 12-hour session the other day, punchy lawmakers shrugged aye to a whole set of sweeping changes in the state's income tax structure. Now, in the harsh light of the morning after, many of the more election-phobic members who gave the nod to this measure may be having second thoughts about it -- and well they should.

It is not that changes in the tax laws should be verboten, or that this bill is totally without merit. Its sponsor, Del. Johnny Joannou (D-Portsmouth), sold the measure as giving the state a more equitable tax structure. This, of course, depends on whose equity we're talking about. The proposal would knock out deductions for such items as interest on home mortgages, major medical expenses and charitable contributions, and replace them with a standard $4,500 deduction; and there would be a flat 4.75 percent tax rate on taxable income instead of a graduated 2-to-5.75 percent tax scale.

So what do you get when you add up all these numbers? It amounts to a hefty heist of Northern Virginians, whose home costs are the highest in the state. Nobody should mind a healthy pinch of progress in the tax structure, but neither the House nor the Senate has had the time to consider the full impact of Mr. Joannou's recipe. There are other questions to be considered, also: Would these changes cause a dramatic drop in charitable contributions? Is it wise or even economical to create a whole set of state forms and regulations that bear no resemblence to their federal counterparts?

To whistle this bill into law in the closing days of a short election-year session would be irresponsible. It is up to a sober Senate to set this measure aside in favor of a serious examination of how best to make Virginia's tax laws more equitable; sensible and workable.