Adams died at his home in Stevensville, Md., after a struggle with Parkinson's Disease, said Ellen Globokar, who was his staff chief in the Senate.
Adams, a Democrat who never lost an election, represented Washington in the U.S. House of Representatives from 1965 to 1977 before becoming transportation secretary from 1977 to 1979.
He went on to unseat Sen. Slade Gorton, R-Wash., in 1986, but declined to seek re-election in 1992 after eight women told The Seattle Times that Adams had harassed them.
Adams denied the allegations.
"There was never any harassment. There was never any threat. There was never any of these things that were stated in the article," he said.
A previous sexual misconduct allegation had made Adams one of the Senate's most vulnerable Democrats. Adams also denied that allegation.
Adams, a former federal prosecutor, served in Congress for nearly two decades and was a stalwart of Democratic politics for nearly 30 years. A hero to many liberals, Adams had strong support from labor unions, women's groups and minorities.
Former Washington Gov. Mike Lowry, who succeeded Adams in the House, called him "a caring person, very intelligent and very capable and always believing in the good things about government service."
Adams distinguished himself as transportation secretary and as chairman of the House Budget Committee, "both real recognitions of his tremendous ability," Lowry said.
"Adams was a giant when it came to addressing the complex transportation needs of the region," said state Democratic Chairman Paul Berendt. "He was instrumental in building up the aerospace infrastructure of this region, as well as our ports and our dominance as a trading center."
After graduating from the University of Washington and Harvard Law School, the Georgia-born Adams worked as a lawyer in Seattle before being elected to Congress in 1964. During his six terms representing the Seattle area, Adams rose to chairman of the House Budget Committee before accepting President Carter's offer to head the Transportation Department.
He served two years in the Cabinet, where he made enemies in the airline industry by opposing deregulation. Adams said it would hurt consumers. He also angered some in the auto industry by pushing for the installation of air bags.
Adams resigned under pressure in 1979, then returned to Washington state to resume his law practice.
He made a successful comeback to politics in 1986, defeating Gorton in a closely contested race. He focused on Gorton's support for military work at the Hanford nuclear weapons complex.
Having served 12 years in the House, Adams rose quickly in the Senate, becoming a member of the Appropriations, Labor and Human Resources and Rules committees.
In his first year, the Democratic leadership turned to Adams to help override President Reagan on a major transportation bill, and Adams filibustered to defeat a bill that would have designated Hanford as a dump site for highly radioactive waste.
The first allegations of sexual misconduct were made against Adams in 1988, when former congressional aide Kari Tupper accused him of drugging her and taking her to bed. Prosecutors said the allegations did not warrant charges.
Adams continued to deny the allegations and prepared his 1992 campaign in the face of criticism from friends as well as enemies.
His withdrawal paved the way for U.S. Sen. Patty Murray, D-Wash., who had already announced a challenge of Adams and went on to win the Senate seat in the so-called "Year of the Woman."
Adams is survived by his wife, Elizabeth; four children, Scott Adams of Jacksonville, Fla; Dean Adams of Rosewell, Ga.; and Kokie Adams and Aleen Adams, both of Seattle; and seven grandchildren. He also is survived by a sister, Phyllis Hayes of Seattle.
Funeral arrangements were pending.