Following his Senate term he was widely recognized as a defender of the civil liberties of individuals accused of being security risks during the Eisenhower administration and as a community activist and moderate Republican until his death in 1979.

[5] Cain's terms as mayor were characterized by his enthusiasm and very public approach to governing, including a weekly radio program that was uncommon for the time.

Cain was placed in charge of 29 towns and villages near Naples, Italy, trying to meet the needs of starving displaced civilians caught between the two armies.

[9][10] In March 1944, Cain was assigned to SHAEF headquarters in London, England where he directed the psychological warfare and public relations division of the G-5 Civil Affairs staff.

In April 1944, Cain was approached by political supporters in Washington state to run as a Republican for the open U.S. Senate seat in the 1944 election.

Although Cain was unable to campaign actively, he won the Republican primary and faced popular Democratic Representative Warren G. Magnuson in the general election.

[11] While he was running for the Senate and carrying out his staff duties in London, the Invasion of Normandy and Operation Market Garden had taken place and Cain had missed both of them.

In September, he was assigned to the XVIII Airborne Corps, commanded by Major General Matthew Ridgway, as Assistant Chief of Staff for Civil Affairs (G-5).

[12] Cain participated in the planning for Operation Varsity, the elimination of the Ruhr Pocket, and the Allied push into Northern Germany.

A day later, Cain delivered a speech at the burial of approximately 200 concentration camp victims near the town of Hagenow, Germany.

He was elected to the Senate on November 5, 1946, defeating Democrat Hugh B. Mitchell, an affable, competent, and decidedly uncharismatic campaigner who had recently been appointed to the position, by more than 60,000 votes.

In this campaign Cain first began to raise the allegations of ties to Communist front organizations against Mitchell and other state Democrats.

He was generally considered to be the real estate industry's strongest supporter in Congress and once made an extended speech attacking Time magazine for including him on a list of the "Senate's Most Expendable" members.

[20] As Cain's term in the Senate wound down, he was targeted by the National Democratic Party for defeat in what otherwise looked like a very promising Republican year.

"[23] At the urging of some of his former Senate colleagues, President Dwight D. Eisenhower appointed Cain to the Subversive Activities Control Board, where he served from 1953 to 1956.

He began to speak out against what he believed to be the excesses of the program in a series of speeches to national civil liberties groups, to the point that White House Chief of Staff Sherman Adams and members of the Justice Department considered him disloyal.

A plaque was presented to Cain with the following inscription: "In Tribute to Harry P. Cain / Champion of Human Dignity, Defender of Constitutional Rights in the Search For National Security / From Those Whose Loyalty to Country He Vindicated, and Those Whose Faith in Freedom He Strengthened / Presented at Testimonial Dinner / National Press Club, Washington, D.C. / 23 October 1956.

In January 1964 he testified in a libel trial brought on behalf of John Goldmark, a Washington state legislator who had been defeated partially on the basis of allegations that his membership in the ACLU was tantamount to being a member of a Communist-front organization.

Cain testified that the ACLU had never been on the Attorney General's list of such organizations and Goldmark won a sizable award from the defendants.

[30] Cain became a familiar face on Miami television, hosting and interviewing national political personalities on a weekly public affairs program that the bank sponsored.

He chaired the Florida Citizens for Johnson-Humphrey, the Democratic Party ticket, in 1964, but supported Nelson Rockefeller and then Richard Nixon in 1968 based on his opposition to the Vietnam War.

He championed one of the first indoor smoking bans in the country and other measures ensuring equal rights in jobs, housing and public accommodation.

In 1935–1936, the couple took an extended trip to England and Germany, where they immersed themselves in theater and he studied British banking methods and listened to the colorful orators in London's Hyde Park.