Few agencies of government have faced as many budgetary ups and downs in one administration as the Export-Import Bank under President Reagan.

Destined for the ash heap in the Reagan administration 1981 budget, when it was described as "welfare for fat cats," the Ex-Im Bank was resurrected two years later with prominent mention in the State of the Union address as neccesary to increase sales of American products overseas.

Now, with the United States carrying a record $123.3 billion trade deficit, the Ex-Im Bank is on the block once more. President Reagan's 1986 budget, officially released yesterday, calls for eliminating its $3.83 billion program of direct loans for foreign purchasers of American products.

The administration said the Ex-Im Bank's direct loans are not needed because other governments have agreed to curb their subsidized export loans. In addition, bank officials said interest rates are falling enough so exporters can compete using commercial loans.

Business interests disagreed, and have already met to begin a campaign to get the lending authority back in the budget.

"What you have is an administration that can't keep its eye on the foreign trade situation," said Lawrence A. Fox, vice president for international economic affairs at the National Association of Manufacturers.

"You would think their minds would be concentrated by a $123 billion deficit, but apparently it isn't."

One business executive called the administration's flip-flops on the Ex-Im Bank a sign of "what in polite company . . . would be referred to as incompetence.

"When it comes to the Ex-Im Bank," the executive continued, "this administration is incompetent and doesn't understand how important credit is to selling big-ticket items" such as jet airplanes, nuclear reactors or electric generators.

"The time couldn't be worse to eliminate the major program to help industry remain competative," said Stuart E. Eizenstat, who was chief domestic adviser to former president Carter and now is a Washington lawyer for Westinghouse Corp.

He said that many U.S. sales overseas depend on financing packages that can compete with subsidized terms offered by other countries. He challenged the administration's contention that the direct loans can be eliminated because of "significant progresss" in getting other nations to stop subsidizing their export financing.

He said it reflects the divisions within the Republican Party between conservative populists such as Budget Director David Stockman and Rep. Jack Kemp (R-N.Y.) and the more traditional Republicans in Congress who support the Ex-Im Bank.

The bank used less than half of its lending authority last year -- $1.47 billion -- because the world recession slowed many major purchases, bank officials said. But last year was an improvement over 1983, when the bank loaned less than $1 billion, and 1985 looks better still.

The NAM's Fox suggested leaving the lending authority at this year's level, although other export-oriented industries appeared willing to take a slight cut.

For businessmen, though, the larger question concerns the administration's inability to arrive at a consistent policy over four years on the role of the Export-Import Bank.

"This may be one of the few times in recent history where a program goes from being included in the State of the Union with favorable mention to being eliminated. It's a rather remarkable tour de force, probably unprecedented," Eizenstat said.

"They used to say that the Carter administration flip-flopped. But this is the biggest flip-flop one could see. . . . What it tells American businessmen is, if they want to export, they should use foreign plants and get foreign export credits overseas," Fox said.