Sen. George McGovern (D-S.D.), denying that he is a lonely voice on the liberal fringe of the Democratic Party, fired back at President Carter yesterday, accusing the President of being out of step with "mainstream Democrats."

"The present economic and social policies of this administration are out of step with the platform of 1976, the platform of 1972 and the platform of 1968," McGovern told a breakfast meeting with reporters.

"These are the kind of things that mainstream Democrats have deplored when advocated by Republicans, and I don't see why we should be silent when they are advocated by a Democrat."

Specifically, McGovern criticized Carter administration policies on unemployment, economic stimulus, farm supports, welfare reform, energy and tax reform.

He said it was premature to speculate about a challenge to the President from the liberal wing of his party in 1980, but warned that if Carter "pushes too hard" in the direction he is headed his critics will multiply and he will risk a credibility gap.

The senator's comments were the latest example of the simmering unhappiness of some liberal Democrats with the President. McGovern, the 1972 Democratic presidential nominee and president of the liberal Americans for Democratic Action, focused public attention on that unhappiness in a speech to the ADA last weekend.

Comparing Carter's economic policies with those of his predecessor, President Ford, McGovern said in the speech that it was sometimes difficult "to remember who won last fall."

The President responded at his news conference Thursday, characterizing his liberal critics as "very difficult to please," the kind of people who "if they get 95 per cent of what they want can only remember the other 5 per cent."

What seemed to rankle McGovern most was Carter's attempt to portray him as a lonely voice of dissent within the Democratic Party. When the President was asked at the news conference about the criticism of "a number of liberal Democrats - thinking especially of Sen. McGovern . . ." he replied:

"Well, I would not refer to Sen. McGovern as a number of liberal Democrats . . . I don't think the criticism was justified and I am very thankful it was confined to one person."

McGovern, who spoke in a calm voice and did not appear angry, said yesterday it would be a mistake to suppose that unhappiness with the administration is confined to "a handful of liberal malcontents." He said he had been reluctant to speak out publicly and did so at the urging of some of his Democratic colleagues in Congress.

"It isn't as though I invented this unrest," he said. "It's been there."

The South Dakotan said Carter's news conference remarks reminded him of some earlier Presidents who "tried to minimize and ridicule and isolate their critics, only to find out there are armies out there when it is time to move."

He conceded that other Democrats have not rushed to join him in public criticism of the President, and said it reminded him of the early days of the Nixon administration, when he was chastised for criticizing Nixon's policies in Vietnam.

But McGovern denied that it is too early to criticize the Carter administration, saying its "general direction" is being set and that "if we wait too long a pattern is set and it's too late."

McGovern's major criticisms were that the President has been too timid in moving on a number of tradition Democratic issues, including unemployment, welfare reform and cutting defense spending. "The conservative business community and the economists who speak for them have gotten 95 per cent of what they want," he said.

In the course of his meeting with reporters, McGovern confirmed that twice in 1972 he was approached by people claiming to represent Carter and was told the then Georgia governor was interested in being his vice presidential running mate.

"I thought if I went to a Southern governor, (Reubin) Askew of Florida was preferable," McGovern said. "All I knew about Carter was that he was one of the leaders of the "Anybody but McGovern movement."