President Carter neared the first crucial defense policy decision of his administration yesterday amid signs that he is reconsidering his campaign opposition to production of the controversial B-1 bomber.

Last July, Carter told the Democratic National Committee platform committee, "The B-1 bomber is an example of a proposed system which should not be funded and would be wasteful of taxpayers' dollars."

But yesterday, the President met with a group of congressional supporters of the B-1, one of whom, Sen. Barry Goldwater (R-Ariz.), said afterwards, "I think he's becoming convinced we can't live without the B-1 . . ."

At the Pentagon, sources said that studies commissioned by Defense Secretary Harold Brown to be given to Carter before he makes a decision argue that the B-1 would be the most effective means of maintaining the United States' "triad" defense posture - composed of land-based missiles, sea-based missiles and manned bombers.

In U.S. District Court in Washington, meanwhile, environmentalists argued that the Defense Department had not adequately considered the potential environmental impact of a fleet of B-1s, Judge Thomas Flannery took under advisement their suit which seeks to halt further work on the plane until an environmental assessment is made.

The impression of Goldwater following the White House meeting was shared in part by House Appropriations Committee Chairman George H. Mahon (D-Tex.). Mahon said that while the President gave no indication of what he will decide in the end the fact that he "has not proposed eliminating of all B-1 funds indcates he is open-minded."

Carter has promised to decide by the end of the month whether to go ahead with production of the controversial bomber, which would be the first American combat aircraft to cost more than $100 million a plane. It would cost $101.1 million. The Air Force, which had the support of former President Ford, is seeking production of 244 B-1 bombers at a cost of $24.8 billion.

White House press secretary Jody Powell insisted that the President has got changed his mind about the B-1 because "he has not made a decision."

"Obviously he is willing to be convinced or is open to argument that it should be funded," Powell said in explaining the meeting yesterday with proponents of the plane.

Carter is to meet Friday with congressional opponents of the B-1 in an apparent attempt to demonstrate his often-promised determination to consult widely with Congress on major decisions.

Powell said the options before the President range from "ending it to full-speed-ahead" on production. While professing ignorance of Carter's inclinations on the B-1, the press secretary added that "the President's basic learning is against spending money" and that, as in most new programs, "the burden is going to be on the advocates to convince him" the bomber is necessary.

There are currently three B-1 test models flying and a fourth is in production. President Ford ordered the first three combat versions built in fiscal 1977 and requested funds for eight additional combat models for the fiscal to five planes that would be built at a cost of $1.4 billion.

The Air Force argues that the new supersonic bomber is essential because the nation's fleet of 25-year old B-25s is growing obsolete. Opponents argue that the age of all manned bombers is nearing an end and that even if a need for bombers continues there are cheaper ways to meet it than the B-1.

Within the range of options the White House is considering, Goldwater, speaking to reporters, suggested a possible compromise. He said the Air Force has not made "an iron-bound case" for building 244 of the bombers and that 155 B1s "could satisfy the strategy needs of the country."

Although Powell insisted that Carter is not backing off from his campaign promises about the B-1, the impressions of Goldwater and Mahon that he is seriously considering approving production of the plane were in sharp contrast to some of the President campaign statements.

In a position paper from his primary campaign, for example, Carter said that while he approved of continued research and development of the B-1 "I oppose production of the B-1 meets this need at this time."

As recently as a news conference last February, the President expressed "serious questions" about continuing the B-1 programs.

But the White House-produced "Promises" book, which lists Carter's major campaign pledges, makes no mention of the B-1.

Goldwater, the 1964 Republican presidential nominee, said he thought he understood what has happened to the President.

"Naturally, when a man is running for office, as I found out, you make most any statement to get the eyes of Carter said. ". . . Then, when you sit down in the quiet of evening and realize what you're said, you change your mind here and there . . . And that's what makes a good President."