President Carter threatened to veto his own tax cut and "reform" legislation if Congress adds provisions reducing taxes on capital gains.

"The American people want some tax relief from the heavy burden of taxation on their shoulders, but neither they nor I will tolerate a plan that provides huge windfalls for millionaires and two bits for the average American," the president told a nationally televised press conference.

Carter has already retreated on his original tax proposals, both in terms of the size of the tax cut - originally $25 billion, since slashed to $20 billion and likely to end up closer to $15 billion - and in the number and scope of the "reforms" in the bill. Yesterday, he conceded that the legislation, as it not stands, "contains no major reforms at all."

Calling that situation "bad enough," the preisdent drew the line, saying that while he might be willing to accept little or no "reform" this year he would not tolerate "a step backward" in the form of provisions that he charged would benefit only the rich.

In effect, his opening statement at the news conference represented a shift in [WORD ILLEGIBLE] for Carter, from a posture of advocating new "reforms" to a defensive position fending off what he considers retrogressive changes.

During the news conference the president also:

Reiterated his support for a "comprehensive" national health insurance plan, but said that because of inflation and "very tight budget constraints" it may take "many years" to put such a plan into effect. But after the news conference, Carter met with one of the chief supporters of national health insurance, Sen. Edward M. Kennedy (D-Mass.), who quoted him as saying he would propose a "comprehensive and universal" health plan.

Called for the resignation of David Gartner, his own appointee to the Commodity Futures Trading Commission, who has received $72,000 in stocks from the chairman of Archer-Daniels-Midland Corp., a major Minneapolis-based grain dealer. (Details on Page E1.)

Carter directed his tax critcism at the so-called Steiger amenment, named after its sponsor, Rep. William A. Steiger (R-Wis), which is being considered as an addition to the tax bill by the House Ways and Means Committee. The amendment would, in effect, reduce the maximum tax on capital gains from 49 per cent to 25 per cent.

In response to a question, the president extended his veto threat to a milder proposal, sponsored by Rep. James R. Jones (D-Okla.), that would cut the maximum tax on capital gains to 35 per cent.

"It's sad that the president of the United States fails to understand our economic system and instead falls back on simplistic rhetoric," said Steiger. He called the veto threat "a bluff."

Jones said, "I think the president is getting bad advice. [Ways and Means Chairman] Al Ullman would not lend his support to the compromise if he didn't think it was fair and in the best economic interest of the country."