THE BACKGROUND music subliminally heard in the Senate finance tax subcommittee the other day could only have been the Harold Arlen-Johnny Mercer oldie, "Ac-cent-u-ate the Positive." The hearing concerned S. 352, a measure to nullify a 1980 IRS "private letter" ruling that denied tax credits for political contributors waging "negative" campaigns. The IRS ruling, complained John T. Dolan, chairman of the National Conservative Political Action Committee (NCPAC), had been issued against his group's "Target '80" effort to bring down incumbment liberal Democratic senators.

Understandably, Robert F. Bauer of the Democratic Senatorial Campaign Committee was there, urging the subcommittee to "eliminate the negative" -- to reject S. 352 and retain the ban on tax credits for "negative" ads. Since individuals could still donate to all political groups, even those waging negative campaigns -- though without tax advantage -- Mr. Bauer denied that the ruling inhibited First Amendment rights. Tax credits, he argued, should not subsidize "the new wave of hate campaigns."

You can share Mr. Bauer's dismay and still believe, as we do, that Mr. Dolan had a telling response. Politics ain't bean bag, he said in essence: those who distinguish between "positive" and "negative" political speech should recall the late Justice Harlan's caution that "one man's vulgarity is another man's lyric." Supporting his position was a constitutional odd-couple, the ACLU and the conservative Washington Legal Foundation, both challenging the IRS for discriminating unconstitutionally against legitimate modes of political expression.

Why should the major parties be allowed to run the roughest of ad campaigns on tax-creditable funds while political action committees -- including those now being belatedly organized by Democrats -- bear the burden of IRS's Higher Law of political behavior? Surely the IRS has better work to perform than calibrating the rhetoric of politicians and their organizations to separate out "positive" and "negative."