More than half the money raised in U.S. credit markets this year will be borrowed by the federal government, as the private sector draws in its horns and the government's borrowing needs swell, according to new estimates from the Office of Management and Budget.

This will be the first year in which the government's share of the total funds raised in U.S. credit markets top the 50 percent level -- 56 percent, according to the estimates. In fiscal 1981 it totalled 34.8 percent, the OMB said, and the new estimates show it dropping back to 46 percent for fiscal 1983. However, that figure assumes that Congress cuts next year's deficit to just over $100 billion.

A major reason for the large anticipated jump in the government's share of credit markets is that the budget office forecasts a big drop in total funds raised in the U.S. markets, from $408 billion in fiscal 1981 to $368 billion in the current fiscal year. This is presumably a result of today's deep recession, which discourages individuals and businesses from borrowing more than they have to.

Another is the sharp rise in the federal deficit, which the administration says has come largely because of unforeseen developments in the economy.

Budget Director David Stockman yesterday questiond the wisdom of locking in long-term tax and spending policies, a key feature of President Reagan's economic program, as this makes it harder to respond to unexpected events.

Stockman told reporters at a breakfast meeting yesterday that he has no doubts about the president's "fundamental program." Stockman predicted lower interest rates and an economic recovery later this year, with real growth of 5 percent "easy to imagine" for a couple of quarters.

But when asked whether the events of the past year suggested that it was a bad idea to have an inflexible, long-term fiscal policy, Stockman replied, "That's a pretty perceptive comment."

He also said that "a lot of garbage got thrown into the pot" of last year's budget and tax plans.

The OMB director did point out that the tax cut enacted last summer meant that "we finally have Congress in a position where it has to make a choice" between cutting spending or voting for tax increases.

Stockman said in answer to a question that he thought "on increasingly frequent occasions" of how nice it might be to leave the administration for quieter pastures.

However, he added jokingly, "I don't think we ought to be predicting either interest rates or my tenure . . . they are both heading in the right direction."

Interest rates likely will fall in coming months as the economy improves, Stockman told reporters. The economic indicators will "look a lot better in October" just before the election than they do now, he said.

He predicted a "fairly healthy consumer-led recovery" in the fall, which would boost business cash flow, reduce the amount of short-term "distress" borrowing that companies are now forced into just to stay afloat, and bring interest rates down.

Some analysts fear, on the contrary, that unless money policy is eased, there is likely to be a further rise in interest rates when the economy picks up.

Stockman said the large prospective federal deficits remain a "roadblock" to sustained economic growth, and he held out the prospect of further spending cuts next year and in later years.

You are "not going to do it all in one year, in one budget . . . in one legislative stroke," but the budget "has to be brought under control," he said.

As House members struggle to come up with a budget plan that can muster a majority, Stockman said he believes there is a "fairly decent chance" that a package will pass the House.

Referring to the present recession as a "correction, a recession," Stockman said it is "unfortunate, painful, difficult, but unavoidable."