At the time of his retirement, he was the most senior member of the Senate, a feat which would be repeated by his immediate successor Patrick Leahy.

As governor, Aiken battled the New Deal over its programs for hydroelectric power and flood control in Vermont.

In the 1960s and 1970s, he steered a middle course on the Vietnam War, opposing Lyndon Johnson's escalation and supporting Richard Nixon's slow withdrawal policies.

As acting chairman of the Senate Agriculture Committee in 1947, he opposed high rigid price supports.

[5] Aiken developed a strong interest in agriculture at an early age, and became a member of the Putney branch of the Grange in 1906.

[7] As Speaker, he shepherded the passage of the Poor Debtor Law, which protected people who could not pay their obligations during the Great Depression.

[6] Aiken earned a reputation as a moderate to liberal Republican, supporting many aspects of the New Deal, but opposing its flood control and land policies.

To continue the effort to form a consumer-oriented PSC, he named the former head of the Vermont Farm Bureau as its chairman.

[14] When only Vermont and Maine voted Republican in the 1936 presidential election, Aiken thought he was in a good position to exert national leadership in the GOP.

[6] During the 1940 presidential campaign, however, conservative Republicans favored Senator Robert Taft of Ohio, liberals were behind New York County District Attorney Thomas Dewey, and the media was enthusiastic for Wall Street tycoon Wendell Willkie, so Aiken's nascent campaign went nowhere.

[15] During his administration, Aiken reduced the state's debt, instituted a "pay-as-you-go" road-building program, and convinced the federal government to abandon its plan to control the Connecticut River Valley flood reduction projects.

[3] He also encouraged suffering farmers in rural Vermont to form co-ops to market their crops and get access to electricity.

He portrayed himself in populist terms as the defender of farmers and "common folk" against the Proctor family and other members of the conservative Republican establishment, and with Ernest W. Gibson and Ernest W. Gibson Jr. became recognized as a leader of Vermont's progressive Republicans, which came to be known as the party's Aiken-Gibson Wing.

Aiken defeated Flanders in the GOP Senate primary in 1940 and was easily elected that fall to complete the remainder of Gibson's term.

The younger Gibson served as a caretaker Senator until January 3, 1941, but did not run in the election to fill the vacancy.

They in turn blocked Aiken's appointment to the Labor and Public Welfare Committee and persuaded conservative leader Robert A. Taft to chair it.

A north-south avenue on the west side of the public lawn at the Vermont State House has been named for him.

When Leahy retired at the end of the 117th Congress in January 2023, the two had held Vermont's Class 3 seat for a combined 81 years, 11 months, and 24 days.