Instead of Dear Rosty letters from the public, Dear Andy letters need to be written. Andy is Rep. Andrew Jacobs, the Indiana Democrat who, as a 10-year member of the House Ways and Means Committee, has worked as conscientiously as anyone in getting fair tax laws written and unfair tax breaks blocked.

Unlike Rep. Dan Rostenkowski of Illinois, an uninspired go-along Democrat who is chummy on and off the golf course with the give-us-a-loophole trade associations, Jacobs has an original mind and an independent bent. He believes the outsiders -- the poor, the working class and others who are threatened by the high costs of getting by -- deserve lighter tax loads, not the rich who are getting them.

Jacobs has been arguing this case for years. But on May 30, during hearings before the Ways and Means Committee, he needed less than three minutes to show that under the Reagan administration's new tax proposals the biggest winners are the wealthy. James Baker, the secretary of the Treasury, was before the committee, and it was in a brief exchange with him that Jacobs most earned the tax dollars that pay his salary.

The congressman told Baker that he paid $79,975 in taxes in 1984. "Under your program," Jacobs said, "I would have paid $65,842, or a reduction -- money in my pocket by your program -- of $14,133." Jacobs is not one of the millionaires of Congress. His tastes run in the opposite direction, toward a Franciscan spirit that prompts him to turn back to the U.S. Treasury 20 percent of his congressional salary.

Jacobs had a large tax debt in 1984 due to a capital-gains assessment from the sale of property in downtown Indianapolis that he purchased from his father 18 years ago. The piece of land, bought by a developer for an office building, brought a gain of $275,000. Because of the size of this 1984 deal, Jacobs describes himself economically as "a king for a year." Had he a taste for the royal treatment given the rich by the Reagan tax plan, Jacobs would have expressed thanks at the hearings to Baker.

Instead, he had a question: "Believing as I do in the doctrine of Will Rogers -- that it is a great country but you can't live in it for nothing -- I wonder where I should stand on the new plan ? From here, even looking at myself, it looks unfair in its application to me with respect to my fellow citizens."

The unfairness is obvious to others. The Center on Budget and Policy Priorities, a nonpartisan Washington research group, reports that "the biggest gainers under the administration's new tax plan are America's wealthiest individuals -- those with incomes over $200,000 a year. The 441,000 taxpayers in the $200,000-plus income bracket would receive an average tax reduction of $9,254 apiece . . . By contrast, persons with incomes below $30,000 a year would realize average gains of less than $150 a year. Middle-class Americans in the $30,000 to $50,000 range would receive average tax reductions of a little more than $200 a year."

Jacobs had more. He asked Baker about the administration's plan to drop the current $1,500 a year exemption given to parents who adopt handicapped or other children with special needs. Why, Jacobs wondered, has the administration found the tax code a suitable place to give exemptions to oil, coal and fuel corporations for the discovery of energy, but not a place to support the adoption of handicapped children? Do you believe it should be that way? Jacobs asked Baker.

"Yes, sir, I do," answered the secretary.

Jacobs reports that "a collective gasp" was heard from many of the 300 people at the hearings. It would figure. It is not often that a public official puts it directly on the record that taking care of the oil companies means more to the government than taking care of handicapped children. The skill of Jacobs -- not shared by Rostenkowski, the willing broom as Reagan sweeps a wider tax path for the privileged -- was that he could clarify the issue with only one or two questions.

At the hearings, the congressman spoke to the unfairness by which huge corporations earn billions of dollars in profits but pay no federal taxes. Jacobs inquired: "As a consequence of your tax program which you got through Congress in 1981, General Electric accumulated tax free a profit of $6.5 billion . . . If your minimum corporate tax had been part of your package in 1981, how much would they have paid?"

Baker didn't know. He would supply it for the record, though, and then guessed that GE would have paid tax. By the end of last week, which was 21 days later, Treasury had yet to come up with a figure. Jacobs may have to depend on the Wall Street Journal for an answer. On June 18, it reported that General Electric was among the military contractors that "emerge as big winners" in the Reagan tax plan.