The Department of Interior, which only last summer endorsed the $46- to 50-million memorial to Franklin Delano Roosevelt proposed for the Tidal Basin's western edge, has reversed itself and now opposes the memorial as extravagantly expensive both to build and to maintain.

In a statement that one key congressional official sees as probably "killing the memorial," Interior Secretary Cecil D. Andrus has called its projected construction and maintenance costs "remarkably high for any memorial." He said Interior now "doubts very seriously" whether a $50-million memorial is "an appropriate tribute... (for a president) who made it very plain that he did not want a large costly monument erected in his honor in Washington.... We did not propose it, and we do not support it."

Although Undersecretary of the Interior James A. Joseph told a congressional committee last July that the proposed memorial had the Carter Administration's approval, an Interior spokesman this week said "that was only a lukewarm endorsement." Interior now is "not just backing away from the memorial. We're turning our backs on it completely," he said.

The major reason for the switch, he said, is that things looked different economically last year, and Interior and its National Park Service, which would maintain the monument, are now faced with severe budget cuts.

The approval of the memorial's general design last year by the Commission of Fine Arts now also appears to have been lukewarm. Fine Arts secretary Charles H. Atherton said last week, "We've never been married to this concept... although we approved it... but I don't think anybody is giving it very high odds" any longer.

The monument proposed by the 24-year-old FDR Memorial Commission, is the first to be approved by Fine Arts. Two previous memorial designs -- one dubbed "instant Stonehenge" and the other "stone darts" -- were rejected because they would detract from the nearby memorials to Lincoln, Jefferson and Washington. The present plan, designed by San Francisco landscape architect Lawrence Halprin, calls for a meandering 1,000-foot granite wall, 12- to 14-feet high around the western edge of the Tidal Basin, with waterfalls, fountains and sculpture.

It would be the largest and costliest memorial ever built in this country. It also would be the most expensive to maintain, with operating costs estimated between $1.2 and $1.6 million a year, including funds for a permanent staff of 45 to 49 and a parttime staff of 24 -- roughly twice the funds and staff needed to maintain the Lincoln and Jefferson Memorials here. The FDR monument's electric bill alone would range from $500,000 to $800,000 a year, at today's electric rates, to run the memorial's many water pumps and lights. The lower figure might be achieved if pumps and lights were turned down or off at night, according to Park Service estimates.

While the proposed memorial has yet to be authorized by Congress, the FDR Commission is a creature of Congress, composed primarily of congressmen, and has been cordially received in its many appearances before congressional committees.

A joint resolution authorizing the project, but not funding it, was approved in the House during the last session of Congress -- and reintroduced again last week. But it died last session in the Senate Rules and Administration Committee, where committee Chairman Sen. Claiborne Pell (D-R.I.), a friend of the Roosevelt family, said "while nobody is more enthusiastic about a Roosevelt Memorial... the cost of $50 million bothers me. This will make this about the most expensive memorial we have ever built, and for us to go into it at this time causes me some hesitation."

When Pell and other congressmen asked if the memorial could be scaled down both in size and cost, FDR Commission chairman, former New York Congressman Eugene J. Keogh said it would be impossible without completely redesigning the project and going back to the Fine Arts Commission and public hearings.

Keogh said this week he would make no comment on the withdrawal of support for the memorial by the Department of Interior but did say, "Over the years we've had alternate proposals, adequate and reasonable proposals, and someone always says, 'Roosevelt's entitled to a great memorial but not this one.' This could go on forever."

Keogh was in Washington last week to complain to a House committee that the White House Office of Management and Budget (OMB) had cut his commission's 1980 operating budget from a requested $50,000 to $10,000. The commission is getting $20,000 this year -- for a full-time secretary and travel expenses for its 12 members -- but had asked for more next year because it had assumed Congress would approve the FDR memorial.

Interior officials explained the proposed OMB cut was made because the FDR Commission still has unspent money from previous years.

Although Interior Secretary Andrus now opposes the FDR memorial project, he added in his statement that "I do understand the desire to celebrate FDR's 100th birthday in 1982, and I pledge cooperation in any reasonable plan to honor him, perhaps by renaming an existing park in the nation's capital. As a leading conservationist and celebrated planter of trees on his own estate at Hyde Park, N.Y., he surely would have believed such recognition appropriate."

Naming the recently created Constitution Gardens in honor of Roosevelt, or in honor of both FDR and his wife Eleanor, has been suggested by citizens and some Interior officials ever since the park was opened during the Bicentennial. Atherton of the Fine Arts Commission last week said he thought the commission would look favorably on such an idea. "I can't see anything wrong with it."

Keogh said, "It's been mentioned one or two times but never given any consideration by the FDR Commission because it came too late," referring to the commission's commitment in 1975 to the Halprin garden wall memorial.

Renaming Constitution Gardens also was one of the more popular proposals last fall at the only public hearing ever held on any of the proposed FDR memorials. The hearing, part of an environmental impact statement Interior is still completing on the proposed monument, was attended by fewer than 50 persons, all of whom opposed the huge memorial. Most citizens and groups who have written to the Park Service, Congress and the FDR Memorial Commission also have opposed it.

While most of the opposition has been to the monument's size and the cost of construction and maintenance, some complaints have been over its location. The meandering wall would circle the western edge of the Tidal Basin, about half way between the Jefferson and Lincoln Memorials, closing one park road and limiting traffic and parking in that area of the Mall. The FDR Commission and Halprin have said in proposing the location this "is the precise site designated in the 1901 McMillan Plan for Washington as a location for a major memorial."

But according to the Fine Arts Commission, the McMillan Plan does not designate any memorial for the area that now is West Potomac Park. "It's an idea we ought to disabuse," says Atherton. "The McMillan Plan in no way contemplated a major memorial (there)... and I've never understood why the architect has emphasized it." The McMillan Plan once projected avenues through that section of parkland, with a traffic circle "where a small monument of some kind might have been built," said Atherton, "but no major memorial ever was planned there."

Washington already has a small memorial to FDR, a plain block of marble the size of the President's desk, placed in 1965 at Pennsylvania Avenue and 9th Streets NW. This is what Roosevelt reportedly told Supreme Court Justice Felix Frankfurter he wanted for a memorial here.

However, a $4-million memorial to the 32nd President also has been proposed on Roosevelt Island in New York's East River, opposite the United Nations. That project, to be paid for by New York City and private funds, was proposed in 1974 but postponed because of the city's financial crisis. The proposed FDR memorial here would be paid for entirely with federal funds.