William Proxmire is winding up three decades in the Senate by still lashing out at big spenders - even among his congressional colleagues - and issuing portents of dire trouble from huge debts hanging over the nation.

As the 100th Congress enters its final days, Proxmire, the outspoken, 72-year-old chairman of the Senate Banking, Housing and Urban Affairs Committee revels in his role of penny-pinching custodian of the public coffers.The Wisconsin Democrat's proudest accomplishment as one of Congress' most prominent mavericks is "holding down spending," he says.

As the Senate opens its daily sessions, the still-tireless senator - known for his 4-mile morning runs to Capitol Hill - is often the first to address the often-empty chamber. Recently, he called for halting the "corporate-raider explosion" and ending spending by "Uncle Sugar" on the space station, which he dubbed a "space palace." "Encourage private enterprise in Houston" to do the job, he advocated.

Alone among the hundreds of House and Senate Democrats, he recently received another "taxpayers' friend" award from the National Taxpayers Union, a group that battles attempts to extract money from the government purse.

"When it comes to controlling spending, no one is close to Sen. Proxmire," said David Keating, executive vice president of the organization. "He has gotten the award for the past 10 years, and we are going to miss him."

But Proxmire's role has hardly been confined to quixotic battles against federal spending. As banking chairman for eight years - from 1975 to 1981 and again after the Democrats regained control of the Senate in 1987 - the senator has overseen the tumult in the financial industries caught up in high inflation, as well as two recessions and finally last year's stock market crash.

In a farewell interview recently in his spartan Capitol Hill office, Proxmire said that his biggest disappointment in his 30 years on Capitol Hill has been not being more effective at cutting spending. When he won an upset victory in a special election in 1957 for the seat of the late Joseph R. McCarthy, the government ran a surplus of $3.4 billion. He saw the budget reach a record deficit of $220 billion in 1986.

"The big problem that sticks out right away is the enormous deficits, the huge national debt and, in this context - that very few people pay attention to - a massive household debt that is $3 trillion, compared to $2.5 trillion of the federal budget," Proxmire said.

"Household debt has gone up about 90 percent in the last eight years. Business debt has gone to $4.3 trillion. When you have a recession, it will strike harder and there will be more insolvencies and bankruptcies and the federal government will be in a position where it will be very hard to bail them out."

But Proxmire may lose some of his credentials as the taxpayers' friend.

To bring down the huge deficits, he said, taxes are going to have to be raised next year. He recommended opening the 1986 tax overhaul package for some "modest, reasonable increase" in the top individual income tax rate, now 28 percent.

And, he estimates that at least $20 billion in taxpayer money is going to be needed to bail out the thrift industry. "Every month that we postpone it, the bailout will be another $1 billion higher, and we are probably going to postpone it until sometime next year," Proxmire said.

But the ranking Republican on the banking committee, Utah Sen. Edwin "Jake" Garn, disagrees. Garn blames Proxmire for stalling for 18 months in 1985 and 1986 on a bill to help rescue the Federal Savings and Loan Insurance Corp. At the time, Proxmire insisted that the FSLIC rescue be joined by a controversial measure that at least temporarily prohibited creation of new, limited-service "non-bank banks."

"It was 15 months (of delay), and we stopped the non-bank banks," Proxmire said, with a broad smile.

In his closing days on Capitol Hill, Proxmire hopes to make a final mark by winning passage of major banking legislation that would partially repeal the Depression-era law that separates banking from the securities industry and other forms of commerce.

"The bank bill is important not for banks alone, but primarily for people who raise capital in this country," he said. "It will provide more competition. The investing public as well as the borrowing public will be served by it."

Proxmire also urged that Congress try to avoid a stock market crash similar to the one last October by creating by law a federal coordinating group to oversee the national markets.

It would comprise the heads of the Department of the Treasury, the Securities and Exchange Commission, the Federal Reserve Board and the Commodity Futures Trading Commission, the same people whom President Reagan named last year to a temporary group to make recommendations to deal with the crash.

The senator also called for tough new federal laws to check insider-trading abuses by increasing the penalties, authorizing bounties to help catch violators and holding securities firms accountable for the actions of their employees.

Regardless of his important role in writing the laws governing banking and securities, Proxmire will undoubtedly be best remembered for his dogged budget battles. Proxmire has repeatedly voted against appropriations bills - except for those that benefit dairy farmers in his home state.

He vigorously opposed money for some of the nation's chief weapons systems, from the B-1 bomber to "Star Wars," sought cuts in basic domestic spending, from subsidized housing to job training and nutrition funds, and even battled against $736,000 for a third Senate gym.

He considers his successful fight against using federal money for a civilian supersonic transport, similar to the Concorde, to be among the highlights of his career.

In his own political races, Proxmire has lived by the same skinflint rules he has tried to impose on the Senate. In 1976 he reported spending $177.75 on his re-election campaign. In 1982 he was down to $145.10. He won both elections by 2-1 margins or better.