In late April, Sen. Robert W. Kasten Jr. (R-Wis.) introduced a new version of his sweeping tax-reform bill, whose avowed purpose was ending the use of tax breaks to create special economic incentives for businesses and individuals.

The next week, Kasten introduced the Small Business Incentives Act, which would, among other provisions, provide several special tax breaks for small firms.

A Kasten aide said the senator was aware there was a "difference" between the proposals, which some analysts might call inconsistent. But the aide emphasized that tax overhaul was not only the top priority for Kasten, but would probably make special aid for small firms less necessary.

Kasten is not alone in seeming to go in two directions on tax policy. Many other supporters of tax revision in both houses also are pushing legislation to create new tax breaks and use the code for purposes other than collecting money.

While their proposals probably won't turn up as amendments to the revision bill, they show how easy it could be to undermine the momentum toward tax restructuring when even the purest of simplifiers have pet causes they want to pursue through the tax code.

"There is a deep inconsistency which reflects the fact that nobody at a real level will accept using the tax code simply for raising revenue," said Norman J. Ornstein, a political scientist with the American Enterprise Institute. "On both sides, there is an underlying acceptance of the notion that we want government to do things."

For example, Rep. Richard A. Gephardt (D-Mo.), author of a Democratic simplification plan, co-sponsored legislation that would let people defer taxes on money placed in special accounts reserved for job training.

Gephardt, an occasional critic of some of the tax breaks in President Reagan's proposal, toured the country two months ago touting the message that social problems can't be cured by putting new preferences in the tax code. Now, he says in favor of the training accounts, "if we can do basic tax reform, we can add some targeted things."

Ornstein noted that these cases of political schizophrenia are not unique to the halls of Congress: Reagan, who has been on the road defending his proposal, also is pushing tax-free enterprise zones for inner cities and tax credits for tuition to private schools.

Few legislators aren't signed on to one tax bill or another, and examples abound among members of the House Ways and Means Committee. Rep. Beryl Anthony (D-Ark.) supports the Gephardt bill, also-authored by Sen. Bill Bradley (D-N.J.), and tax-deferred savings accounts for housing. Rep. Donald J. Pease (D-Ohio) supports Bradley-Gephardt and curbing deductions for advertising associated with arms sales.

Rep. Byron L. Dorgan (D-N.D.) supports Bradley-Gephardt and legislation to restrict tax benefits for companies involved in large mergers and takeovers. Rep. Ronnie G. Flippo (D-Ala.) supports Bradley-Gephardt and tax credits for solar energy.

The congressmen's explanations for the seeming contradictions vary, but they say their support for the other bills does not lessen their commitment to trashing tax breaks and reducing rates.

Pease says it probably isn't consistent to support tax incentives and tax simplification, but points out that he is only a co-sponsor, not the principal author, of the other bills.

"Co-sponsorship doesn't indicate much of anything except in general you think it's not a bad idea to do this or that," Pease said.

In the Senate, Bradley, concerned about diluting the focus on tax revision, decided not to sponsor any tax measure but his own overhaul bill this year, a spokeswoman said. But other supporters of his measure can be found advocating several tax-fiddling proposals.

For example, one Bradley-Gephardt supporter, Sen. John F. Kerry (D-Mass.), also favors a proposal to let faculty members rent housing from universities at low cost while not paying taxes on the value of the difference between actual rent and market-value rent.

The list goes on in both houses: Some 950 tax bills have been introduced in the House this year, and about 150 in the Senate.

Sen. Gary Hart (D-Colo.) is behind individual training accounts, energy credits and Bradley-Gephardt. As a presidential candidate, Hart faced the same contradiction, advocating numerous tax-code methods of dealing with social problems while pushing simplification as well.

A spokeswoman said that Hart considers reshaping the tax code the most important priority, and that he is pushing for other bills that achieve social goals in non-tax ways.

"While we have this tax system, we have to assume that until it changes, one can use it for purposes of stimulating the economy," Hart said.