Reagan's big selling job: a new tax plan. Middle Americans want loopholes closed but don't want to lose individual deductions

President Reagan's tax plan may strike a responsive chord in middle America. A wide variety of Americans, it seems, are interested in tax reform. For example:

Mark Schulz, a farm veterinarian from Hartley, Iowa, talks long and eloquently about restoring fairness to the tax code. Last week, after treating a farmer's pigs for 45 minutes, he spent an extra hour in the hog house talking reform with the farmer. ``He was real receptive,'' Dr. Schulz recalls.

When Jerry Gibson isn't counseling business clients in Stow, Mass., he's polishing up his tax-reform proposal. ``It would save a lot of money for a lot of people,'' he says. Recently, the plan was sent to congressional leaders.

Richard J. Dennis, a Chicago commodities broker, figures he would pay an additional 3 percent in taxes under the Bradley-Gephardt reform measure. ``Nonetheless, I'm all for it,'' he wrote in a recent New York Times editorial. ``. . . I'd be willing to pay a little more to get a fair and simple system that would treat all taxpayers equally.''

Is this a tax revolt? Well, no, opinion experts say. A surprising majority of Americans feel they pay their fare share of taxes. But, these experts add, there does seem to be broad concern that many individuals and companies are not paying their fair share.

``There is a popular, pervasive demand out there'' for fairness, says Bill Vaughan, administrative aide to Rep. Fortney H. (Pete) Stark (D) of California. ``People who two years ago would not have mentioned a tax reform, [now] it's in Paragraph 3 of their letters.'' The message: ``I want everybody to pay something.''

Last fall, a tax reform group released a study that showed 128 large US companies paid no taxes in at least one year between 1981 and 1983.

People are upset about that, says David Wilhelm, executive director of Citizens for Tax Justice, which published the report. The nonprofit research organization, based in Washington, D.C., is working with union, citizen, and church groups on a nationwide petition drive in support of a minimum tax for corporations. In 11/2 months, an estimated 70,000 signatures have been turned in to his office, Mr. Wilhelm says. The groups hope to get 1 million signatures by Labor Day.

In fact, some union locals of the American Federation of Government Employees have reported 5 percent gains in membership because of their support for the petition, says Arne Anderson, an economist and writer for the union's legislative department. The issue ``is taking root wherever we go with it.''

Public interest in tax reform has also been heightened by the attention given it in Washington, D.C., says Everett Ladd, executive director of the Roper Center for Public Opinion Research. While much has been made about differences between the various proposals, there is surprising agreement among experts that more people need to pay into the system at reduced rates. ``It's just not a few conservative economists but a fairly large body of public opinion.''

Often, the public's agenda is set at the top, Dr. Ladd adds. But whether President Reagan's plan can capture the public's imagination is unclear.

``The issue is gaining [in] consciousness,'' says Paul Harstad, senior analyst for Hart Research Associates Inc. But ``it remains to be seen if the public will be galvanized.''

One reason is that the present system in general commands fairly widespread support.

``This present messy, complex, contradictory system, you know, has achieved a remarkable degree of acceptance,'' Ladd says. In January, for example, a CBS News/New York Times survey found that 51 percent of those polled felt they paid their fare share in taxes -- remarkable for something as distasteful as taxes, he adds.

Another obstacle to gaining public support is the complexity of the issue.

``It is very nebulous,'' says Mr. Harstad of Hart Research.

The idea of reform means different things to different people, Mr. Gibson adds. ``Each person has a special ax to grind.''

The AFL-CIO, for example, is supporting a minimum tax on corporations, but it is fighting tooth and nail measures that would tax employees' fringe benefits. It is not alone.

A Harris poll last December found that even though a large majority of Americans approve of the President's original reform plan, they were often unwilling to give up their tax deductions.

``I'm afraid we have a lot of shortsighted people,'' Schulz says. ``And that's what makes a shortsighted Congress.''

Goals, too, often diverge. Both Schulz and Gibson want to replace the current tax on income with one on consumption -- taxing people on what they buy instead of on what they earn. Schulz favors this because, he says, tax dodgers get favored treatment under the current system.

Gibson's concern centers on the billions of dollars made through questionable or illegal dealings that currently go unreported.