And now the bad news:

Those widely heralded tax cuts that Republican presidential candidate Ronald Regan, and now Senate Democrats, are promising for 1981 actually would not reduce Americans' taxes.

In fact, they would only partly offset some of the tax increases that otherwise would occur either because of changes in the law or from the impact of inflation in pushing taxpayers into higher brackets.

Whatever tax-cut plan ultimately is enacted, the tax burden for most households -- the percentage of income that taxpayers give the government -- will remain about the same, and in many cases will rise.

As Reagan and other Republicans delight in pointing out, the only way President Carter and Congress were able to balance the fiscal 1981 budget -- as they have done precariously for now -- was by allowing this tax burden to rise.

Between 1980 and 1981, the impact of inflation is expected to add about $23.4 billion to individuals' income-tax bills. An already scheduled increase in Social Security taxes next Jan. 1 will raise those levies $13.7 billion.

Between 1979 and 1980 -- the last big tax cut took effect in January 1979 -- inflation added $17.1 billion to Americans' income-tax burdens. Thus, the total tax hike for the two years combined is $54.2 billion.

But the tax cuts either Reagan or the Senate Democrats are considering would barely offset even the tax rise for 1981, let alone the $7.1 billion increase that has taken place this year.

Reagan's new plan would trim individuals' federal income-tax bills by $31.8 billion during calendar 1981, and while the Democrats' proposal is not fully worked out, insiders say it will be smaller than the GOP cut.

President Carter's top economic advisers have been discussing a possible $20 billion to $30 billion tax-cut plan that would reduce individuals' income taxes by between $13 billion and $18 billion, with the rest for business.

Although the calculations are only preliminary, Reagan's tax-cut plan would merely hold the average tax burden to roughly the same 17.5 percent of income it is now. Without this cut, the burden would rise to 18.8 percent. policy-makers are considering -- would permit the effective tax rate to rise to nearly 18 percent. The plans of the Senate Democrats are still too fuzzy to measure.

The mathematics of election-year tax-cutting is well understood by both Congress and the White House, but enough voters still seem unaware of the sleight-of-hand arithmetic for the officeholders to keep using it again and again.

Inflation provides the magic in the tax machine. Every year it pushes Americans' tax burdens higher. Then, almost as routinely, Congress gives the money back -- with a tax cut just before the election.

But this year, the lawmakers -- responding to heightened demand for inflation-fighting -- chose to forego the traditional election-year tax cut and try to balance the budget instead. As a result, tax burdens have continued to climb.

At the same time, Social Security taxes are scheduled to rise sharply in January. On the first of the year, the maximum payroll tax paid by workers will leap to $1,975.05 -- from $1,587.67 this year.

The budget Carter unveiled last January called for the tax burden borne by individuals and business to soar to 21.7 percent of the gross national product in fiscal 1981 -- the highest level since 1944.

Any tax reductions approved for 1981 wouldn't start showing up in the form of lower withholding taxes until Jan. 1. The reductions would affect the taxes due a year from next April.

Reagan seized upon this reality last week when he proposed slashing income-tax rates for individuals next Jan. 1 to offset at least some of this increased bite.

Republicans have been charging all along that Carter has been trying to fight inflation in large part by raising taxes. And the Democrats know there's a big enough grain of truth in that to set them worrying about the election.

In the meantime, the real battle between the various tax-cut plans proffered by Reagan, Carter and possibly the Senate Democrats between now and Nov. 4 won't be, as all sides will want to portray it, over cutting taxes. a

It'll be over how much to allow them to go up.