So President Reagan finds the flat tax ''tempting.'' Well, join the club. Everybody says a flat tax will never get through Congress. But suddenly everybody - like Mr. Reagan in his most favorable comments so far - finds it worth looking at. The immediate hope is that this new thrust for total tax reform may at least bring some tax reform.
The breath-taking simplicity of the flat tax is what many find most tempting. The President noted that taxpayers are fed up with present complexities. The flat tax would eliminate these by setting one tax rate for all levels of income while getting rid of the dense array of deductions and credits.
The theory is that a relatively low rate -- an estimated 10.9 percent for individuals in 1983 -- would bring in no less revenue because it would be applied to a much broader base of taxable income.
No wonder bipartisan interest in the flat tax -- or some dilution thereof -- has been growing. Possibly Mr. Reagan noticed not only that a number of his fellow Republicans are calling for it but that the Democratic Party endorsed ''a uniform rate for most taxpayers'' at its Philadelphia conference. The Democrats' final position statement did mention leading a national debate over reform alternatives in addition to the flat-rate tax. And such debate there should be.But their stress was on that uniform low rate for most taxpayers and elimination of most credits, exclusions, and deductions.
The Democrats tried to preserve their franchise on concern for the present lower brackets by addressing a main criticism of the flat rate: that it favors the rich. They would provide an ''additional progressive rate for those in the upper income groups.''
Flat-tax purists say that all such departures undercut the overriding virtue of simplicity.They argue that to have more than one bracket could be counterproductive. For example, it could perpetuate the present incentive for taxpayers to shift income to cheaper brackets through such means as siphoning it off to children.
Yet there is a view that simple, across-the-board solutions can bring inequity; that some of the intricacies of the progressive system arose in the effort to be fair. Substantial standard exemptions have been suggested as one way to make a flat tax more equitable for lower income groups. Retaining some deductions, such as mortgage interest or charitable deductions, has also been proposed.
Mr. Reagan has previously expressed concern about giving up the deduction for charity. But in his remarks this week he recalled that Americans' generosity did not begin with the age of deductions. He doubted it would dry up under a flat tax system.
He is probably right. But, whether he is or not, something is wrong with a system that invites people to make not only charitable donations but business and personal decisions according to tax consequences.
The Senate Finance Committee's new tax bill, for all the tax breaks it preserved, was seen to reflect a senatorial desire for basic tax change. Change toward simplification, lower rates, and broadening the tax base through reducing deductions. Some deductions were actually reduced.
Who knows? Maybe even this much progress owes something to the flat-tax breezes at Congress's back.