Peter M. Flanigan, an investment banker who was one of Richard M. Nixon’s top advisers on business and the economy and helped coordinate Nixon’s 1968 presidential campaign, died July 29 at a hospital outside Salzburg, Austria. He was 90.

His daughter Megan Flanigan confirmed the death but declined to give a cause. With his second wife, Dorothea von Oswald, Mr. Flanigan lived in Wildenhag, Austria, and Purchase, N.Y.

After working as deputy campaign manager for Nixon’s successful 1968 presidential run, Mr. Flanigan served as an unpaid consultant in filling top executive branch appointments during Nixon’s presidency. He then joined the White House staff as an assistant to the president, focusing on economic and financial matters. In 1972, he was named assistant for international economic affairs.

In 1972, Time magazine called Mr. Flanigan “Nixon’s ‘Mr. Fixit’ when it comes to powerful business interests.” It said consumer advocate Ralph Nader had called Mr. Flanigan the “most evil” man in Washington because of his record of influencing White House policymaking in favor of business.

In a 1969 profile, the New York Times called Mr. Flanigan “possibly the least known of President Nixon’s dozen or so top aides.” Nixon, in a 1974 letter accepting Mr. Flanigan’s resignation, said he had shown “immense effectiveness and unfailingly good judgment” in his work on the international economic system and on the transition from the draft to an all-volunteer military.

Peter Flanigan, White House economic aide, testifying in Washington, D.C. Flanigan, an investment banker who was a top financial adviser to Richard Nixon, dies at 90. (Margaret Thomas/The Washington Post)

Mr. Flanigan left the administration in June 1974, just weeks before the Watergate scandal forced Nixon to resign the presidency. After Nixon’s successor, Gerald Ford, nominated Mr. Flanigan as U.S. ambassador to Spain, some senators rebelled, citing testimony during the Watergate hearings that suggested Mr. Flanigan had helped arrange the appointment of Ruth L. Farkas as ambassador to Luxembourg after a large campaign donation.

Making Mr. Flanigan an ambassador “will disgrace the U.S.,” said Sen. Thomas Eagleton (D-Mo.).

Ford ended up withdrawing his nomination.

Mr. Flanigan returned to his job on Wall Street with Dillon Read, an investment banking firm. He worked at the firm until 1992.

Peter Magnus Flanigan was born June 21, 1923, in New York. His father was chairman of Manufacturers Trust Co. when it merged in 1961 with Hanover Bank to form Manufacturers Hanover Trust Co. His mother was a granddaughter of Adolphus Busch, co-founder of Anheuser-Busch.

Mr. Flanigan served in the Navy as a pilot during World War II and graduated from Princeton University in 1947. From 1949 to 1950, he worked in London for the Economic Cooperation Administration, the U.S. agency that administered the postwar Marshall Plan. He became a vice president at Dillon Read in 1954, according to a biography on the Web site of the Nixon Presidential Library and Museum.

In 1959, he organized New Yorkers for Nixon, a group supporting the then-U.S. vice president’s first bid for the White House.

When Nixon moved to New York City in 1963, Mr. Flanigan became part of an inner circle that introduced him to new contributors and plotted his political comeback.

Mr. Flanigan’s wife, the former Brigid Snow, died in 2006.

Survivors include his second wife, Dorothea von Oswald; five children from his first marriage; five stepchildren; and 16 grandchildren.