In his State of the Union address, President Reagan extolled the virtues of his 1981 tax cut. "Pushing down tax rates has freed our economy to vault forward to record growth," he said.

What the president didn't mention was the degree to which his cherished tax cut has been eroded by subsequent tax increases to deal with federal budget deficits. As the president's own budget proposal points out, there have been six tax laws passed since enactment of the Economic Recovery Tax Act of 1981.

Without them, total tax revenue would have been $168.5 billion lower in 1985 than without the Reagan cut, the budget proposal says. But Congress reversed course after enacting the 1981 law and passed the euphemistically titled Tax Equity and Fiscal Responsibility Act of 1982. The provisions in that act will add $40.7 billion in taxes in 1985, reducing the benefits of the 1981 tax cut by that amount.

Just a few months later, Congress raised the gasoline tax by a nickel. That translates into a $4.4 billion tax boost in 1985. The Social Security Amendments of 1983 will add an additional $9.3 billion this year, mostly through higher payroll taxes.

Congress reversed directions briefly in 1983, cutting taxes again by repealing withholding of taxes on interest and dividend income, which it had enacted less than a year before. That reduced taxes this year by $2.4 billion.

But the wind shifted again almost immediately when Congress passed the Railroad Retirement Act of 1983 (a $700 million tax boost) and the mammoth Deficit Reduction Act of 1984 ($9.3 billion).

The remaining tax cut in 1985: $106.4 billion, compared with the original $168.5 billion.

But individuals who worry more about their paychecks than their dividend checks shouldn't be too alarmed: The impact of those tax laws was far harder on companies than on individuals.

Individuals actually will get a greater income tax cut, thanks to the post-1981 tax laws. The personal portion of the 1981 cut is $114.3 billion in 1985, before taking account of the subsequent tax changes. But the net effect of TEFRA, HRA, SSA, IDTCA, RRA and DRA contributed another $2.7 billion to the cut.

Individuals thus are paying $117 billion less in taxes thanks to the changes of the past four years. But income taxes aren't the only ball game. Social Security taxes are $14.4 billion higher and workers pay half of that increase.

Corporations, however, finished the first Reagan term barely in better shape from a tax standpoint than when they went in. Although they received a $31.1 billion cut in 1985 terms, much of that was rolled back by the later laws. TEFRA alone racked up $19.8 billion by doing away with what the budget cheerfully calls "unintended benefits and obsolete incentives."

By the time all was counted, corporations were only $10.5 billion better off in 1985 than they were before the 1981 tax cut, having lost two-thirds of their cut. Nor does that include the business share of higher Social Security payroll taxes.