With an election just down the pike and fiscal austerity on everyone's lips, the Senate decided last fall that there simply was no way it could spend millions of dollars on flashy new furniture for the flashy new Hart office building.

The $137.7 million edifice was controversial enough without one more flap over furniture. What, after all, might voters think in a recession? Curmudgeons were comparing the place to the Taj Mahal, and some image-conscious senators were refusing to move into the building.

But that was September. The election came and went. Then a startling discovery was made last week. Senatorial poohbahs found an extra $9.5 million gathering moss in the legislative cash register.

It took no great leaps of imagination to find a way to spend it. The Senate Office Building Commission, an informal panel that makes building decisions, voted to equip the Hart building with that stylish new furniture.

A final decision will be up to the Senate Appropriations Committee, but there is not likely to be any problem there. The chairman, Mark O. Hatfield (R-Ore.), happens to be an advocate of the avante garde modular furniture that would go into the Hart offices.

In fact, Hatfield's new office in the Hart building also happens to be furnished with the very type of equipment being contemplated by the commission for the other 49 suites there. A stream of visitors regularly walks through Hatfield's "model home" to glimpse the future.

Among others, the building commission chairman, Sen. Robert T. Stafford (R-Vt.), looked over Hatfield's suite and evidently liked what he saw: sleek wooden desks and cabinets in large work areas, with key staff members shielded from distraction in cozy little carrels.

Money for the furnishings, for which bids are to be sought, will come from funds left over in the legislative appropriations for fiscal years 1980 and 1981.

The commission made another fortuitous discovery last week. The work of art designed for the Hart building's gaping atrium by the late Alexander Calder, regarded as one of America's premier sculptors, also will come out of the deep freeze.

Nicholas F. Brady, a wealthy Republican who last year served an eight-month unexpired term as a senator from New Jersey, has taken it upon himself to find a way to pay the estimated $400,000 cost of the huge stabile, which Calder named "Mountains and Clouds."

Calder completed the design just hours before his death in 1976. But when cost overruns threatened to stop the Hart building, the stabile was put on hold. There was no way that kind of money could be spent on decor, the senators decided.

Before he left the Senate, Brady had said he would help with the Calder work if his colleagues were willing. The building commission decided to take him up on the offer.

"I told them I would undertake to get it financed privately if the Senate wanted it," Brady said last week. "It seemed endless and fruitless to talk about government paying for it . . . . But the building is built, and it makes no sense to stop the Calder now."

He added, "You look at the well in the middle of that building, and you see it cries out for something; otherwise they'll be playing softball or touch football in there."

Brady said that as soon as he gets official word his help is welcome, he will get on with raising the money. He said some of the financing might come from his own pocket.

When he learned that his former colleagues had found money for their furniture but not for the stabile, all Brady could say was: "For God's sake. Imagine that."

But then, he is not running for anything.