Donald T. Regan, 84, a brash and innovative Wall Street executive who became Treasury secretary and chief of staff for President Ronald Reagan before being forced to resign during the Iran-contra scandal, died June 10 at a hospital in Williamsburg. He had cancer.

Mr. Regan spent his pre-government career with Merrill Lynch & Co. and played a central role in shaping that brokerage house into a successful, diversified financial services business. He was its chairman and chief executive and a noted champion of free enterprise when Reagan tapped him as Treasury secretary in 1981.

During the next six years, as he moved from the Treasury Department to the White House, he found that the elements of his personality that had helped him climb so far began to work against him. His gruff and demanding demeanor -- which he honed as a Marine Corps officer, Wall Street leader and Treasury official -- became a liability as debates erupted over spending cuts, tax simplification and arms control.

The Iran-contra debacle became the coup de grace.

At Treasury, he was the administration's principal advocate for supply side economics, a belief that a reduction in tax rates prompts more people to work and creates a big increase in production. Originally, he did not support the program, but he soon developed the mantra that it was "just old-fashioned conservatism" under a different name.

Regarded as a loyalist and expert tactician, he quickly lobbied a massive tax cut program through Congress, saying it was more essential to the country's economic welfare than balancing the budget. The 1981 Economic Recovery Act, as it was called, resulted in one of the largest-ever cuts in taxes on income and capital gains.

For his work, he earned kudos from conservatives on Capitol Hill, and Reagan called him a "fine salesman."

He spent four more years as Treasury secretary before Reagan named him chief of staff, a position from which he hoped to have greater say on policy in the president's second term. He succeeded James A. Baker III, who swapped the Treasury job with him.

Mr. Regan had more-centralized responsibility at the White House than Baker, who had shared much authority with Deputy Chief of Staff Michael K. Deaver and counselor Edwin Meese III.

"I'll be doing the whole thing now," Mr. Regan told reporters. "I'm not trying to grab power. I'm trying to fill a void."

A star at Treasury -- he was called "the Chief" -- he soon found himself unable to exert supreme control over a staff that he said was riven by leaks to the media and personality clashes.

When he was bold, he often irritated those around him, notably first lady Nancy Reagan.

He once ordered a White House helicopter to take him to National Naval Medical Center in Bethesda, where the president was undergoing cancer surgery in July 1985. After Nancy Reagan expressed extreme displeasure at such a grand gesture -- which he claimed was simply to save time -- he instead went by car.

One way he stayed in Ronald Reagan's good graces was by making sure an assistant armed him every morning with a joke to tell the president.

He said he helped persuade the president to nominate Judge Antonin Scalia to the U.S. Supreme Court instead of Judge Robert H. Bork and pressed Reagan to work with new Soviet leader Mikhail S. Gorbachev.

When reports surfaced in 1986 about the Iran-contra matter, Mr. Regan sustained much of the criticism. He publicly denied intimate involvement in the scandal, a plan to send military equipment to Iran in exchange for that country's influence to help free American hostages held in the Middle East. Also, money from the weapons sales was sent to aid contra rebels fighting the Soviet-backed Sandinista government in Nicaragua.

"By many accounts, Regan was too much like Reagan," David Hoffman, who covered the White House for The Washington Post, wrote in 1987. "He was too reluctant to make hard choices on such vital policy matters as arms control and deficit reduction, he was too quick to paper over problems with salesmanship, he was too passive when it came to the failures of subordinates."

A presidential board headed by former senator John Tower (R-Tex.) found in its 1987 report on the Iran-contra affair that Mr. Regan was largely to blame for the "chaos that descended upon the White House."

"He, as much as anyone, should have insisted that an orderly process be served," the board said.

Mr. Regan, who denied involvement in the scandal, shot back: "Does a bank president know whether a bank teller is fiddling around with the books? No."

Mr. Regan was troubled when, in early 1987, he heard through another administration official that he would be replaced by former senator Howard H. Baker Jr. (R-Tenn.).

He resigned and then wrote an autobiography that emphasized a bitter White House environment. In the first chapter of "For the Record" (1988), he took aim at Nancy Reagan and wrote in especially unflattering ways about the first lady's use of an astrologer to chart the president's schedule.

Nancy Reagan issued a statement yesterday praising Mr. Regan's public service. "I was very sad this morning when I heard of Don Regan's death. From the Marine Corps to Wall Street to the Treasury Department to the White House, Don served this country with great distinction."

After the book, Mr. Regan retired to Williamsburg and took up landscape painting.

"After Wall Street and government, I decided that there had to be more to life than the stock market, golf and drinking," he told The Post in 2000.

Donald Thomas Regan, whose father was a railroad policeman, was a native of Cambridge, Mass. He was a scholarship student at Harvard University, where his major was English but where his talent for business was already on display. He organized a tour guide service that netted him $2,000 by graduation day in 1940.

He left Harvard law school, where he also had a scholarship, to enlist in the Marine Corps during World War II. He served in the Pacific theater and participated in five major campaigns, including Guadalcanal and Okinawa.

By the time he returned from war, he was a married father and believed that law school "was not a practical choice." He instead embarked on a career as a stockbroker and chose what was then Merrill Lynch, Pierce, Fenner & Beane, which agreed to train him for his new field.

Company executives soon recognized Mr. Regan's potential and began grooming him for a top leadership role. He learned all aspects of the securities business during the postwar boom and by 1954 was a general partner.

He later became director of the administrative division, a position from which he oversaw large-scale planning as the company diversified -- into mutual funds and commodity trading, among other areas -- to help shield it from the vagaries of market cycles. He was named president in 1968, the youngest man to hold that position, and chairman and chief executive in 1971.

His earlier corporate vision paid off, as Merrill Lynch became one of the top-grossing companies on Wall Street in the 1970s.

He served as board vice chairman of the New York Stock Exchange from 1972 to 1975, but the consummate businessman was also regarded as a maverick for his public statements condemning common Wall Street practices. He called for killing off the fixed minimum commission, which was said to keep brokers' commissions artificially high and stymie competition.

"Wall Street is hiding behind a protective pricing system while it preaches free competition and free markets," he said in a 1970 speech. "That is like catching Carrie Nation [the Prohibition advocate] tippling in the basement."

The stock exchange rule about fixed commissions was changed in 1975, and Merrill Lynch was among the businesses well positioned to buy firms less able to survive in the new era. Among those it bought, for $50 million, was White, Weld & Co., the legendary banking house.

Mr. Regan made himself a millionaire many times over.

Though a Republican, he was a controversial choice for Treasury secretary for having supported some Democratic candidates over the years and backing President Richard M. Nixon's wage-price control efforts of the early 1970s. One of his greatest backers was CIA Director William J. Casey, a former chairman of the Securities and Exchange Commission.

As Treasury secretary, Mr. Regan was known for giving careful consideration to the most junior members of Congress and for a politically savvy sense of humor.

In 1982, as his legislative strategy group was reviewing the impact of new taxes on alcohol, a few aides agreed that wine was the drink of the rich, beer of the poor.

Mr. Regan said such a sweeping statement was untrue. Then he sent the aides a bottle of $3 wine with a note: "Taste this and see what I mean."

Survivors include his wife, Ann Buchanan Regan, whom he married in 1942, of Williamsburg; four children, Donna Regan Lefeve of Alexandria, Donald T. "Tom" Regan Jr. of Sarasota, Fla., Richard W. Regan of Denver and Diane Regan Doniger of Santa Fe, N.M.; and nine grandchildren.

Donald T. Regan went from Wall Street to Treasury and then became White House chief of staff.