President Carter insisted that he will not back off from his proposed tax legislation, arguing that it will drive down unemployement and increase family income, but will not add to inflation.

Appearing determined to get on his way, the president resorted to some of the style and rhetoric of his 1976 campaign as he called on Congress to correct what he described as "a gross abuse of the average American family" in the tax laws of the country.

Carter complained particularly about the fate of his revenue-raising tax "reform" proposals in the House Ways and Means Committee. He watered down, those proposals himself before submitting them, but the committee has already voted to shelve much of what he did seek.

"On Capitol Hill now," he told a nationally televised news conference, "there is a concentrated and unbelievable number of highly qualified, very intelligent, very effective lobbyists trying to induce the members of Congress to Preserve those special privileges for people who have them because they so powerful and so influential now and in the past that they could carve out for themselves some special deal in the income tax laws of our country at the expense of the average American family.

"That's is where tax reform comes in," he added.

It was popular theme right out of the Carter campaign, invoked by a president who appeared determined to overcome his image as an indecisive and sometimes weak leader.

"No," Carter said bluntly when he was asked whether he could also consider the suggestions of congressional Democrats that he trims his net tax cut proposals from $25 billion to less than $20 billlion and delay the effective date of the cuts from Oct. 1 until next year.

But even as the president was forcefully reiterating his position on national television, on Capitol Hill House speaker, Thomas P. O'Neill Jr. (D Mass) was suggesting that in the end he may have to settle for about a $20 billion tax cut.

Giving some insight into what it is like he told this year but had received no answer in return.

"I thought his silence implied consent," O'Neill said."I learned when he doesn't give me an answer it doesn't imply yes."

The Speaker said that while some House members want to scuttle the entire tax package "the votes are on the floor of the House for the tax cut."

Carter's news conference demeanor appeared to be part of a White Hose plan to project the president as more forceful and certain, one in a series of such steps that have unfolded since Carter huddled with his top aides at Camp David last week.

When he was asked about tax "reform," Carter indignantly cited two examples of "abuses in the system" - a doctor who claimed a $14,000 tax write for entertaining other physicians on his yatch, and a business who claimed deductions for 388 business lunches in one year.

"Most American citizens don't have a yatch and when they go for a small pleasure ride, if they do, have a small boat, theycan't deduct it as an income tax deduction," the president said. "And when that doctor didn't pay his $14,000 in taxes, other average working American families had to pay his taxes for him.

Both cases are included in a Treasury Department document made public when Carter proposed his tax legislation. However, it could not be determined last night whether the Internal Revenue Service allowed the full claims by the doctor and the businessman. In the case of the doctor, a $14,000 exemption would have relieved him of that much by the amount he would owe on his income.

In arguing for the full $25 billion tax cut he has proposed, the president said that if Congress does not enact the legislation "by the end of 1979 it would cost every family in America on the average $600 in income, about $40 billion in reduced income because of a constrained economy that did not continue to grow.

"If the tax reduction of $25 billion was eliminated," he continued, "it would mean, we would have a million more people out of work by the end of the first 12 months after the tax reduction than we would have otherwise."

The 1 million unemployed figure is based on Council of Economic Advisers estimate projecting a 1 percent decline in the gross national product by the end of 1979 without a tax cut. Officials could not be reached last night to explain the basis for the income loss figure used by Carter.

The president's tough talk since the Camp David meeting at times has appeared to put him in the posture of attacking a Congress controlled by his own party.

Asked about this yesterday, Carter claimed that the recent jump in stock market prices may have had something to do with his public stance on inflation and his prodding of Congress to act on his energy legislation.

"I am not attacking the Congress, but I reserve the right to point out the inactivity of Congress which I think on occasion does inspire them to act more rapidly," he said.

The president opened the news conference with a plea for his plan to overhaul the civil service system, which, after the energy and tax legislation, is his highest domestic priority this year.

On other subjects, Carter again defended the higher Social Security taxes he pushed through Congress last year, calling them a "kind of investment" for middle-income Americans. He also said he believes the unemployment rate will continue to fall even as the administration steps up its anti-inflation activities in the short term interest rate by the Federal Reserve Board.