He was the Democratic nominee for U.S. Senate in 1976, replacing Jerry Litton who died after winning the nomination, but lost to John Danforth in November.

Soon after reporting for duty, Hearnes was appointed by President Roosevelt to the United States Military Academy at West Point, Class of 1946.

[12] Hearnes considered running as a favorite son candidate in the 1968 Democratic Party presidential primaries, but ultimately withdrew his candidacy.

He chaired the Missouri delegation to the 1968 Democratic National Convention, which was selected by a traditional caucus of party members and overwhelmingly supported the candidacy of Hubert Humphrey.

[16] Hearnes' priorities as Governor included improving public education, bettering the state's highways and traffic safety, as well as civil rights and the environment.

He also created the Missouri Division of Highway Safety and enacted a law providing mandatory breath tests for suspected drunken drivers.

As governor he also strengthened the Fair Employment Practices Act and increased the staff of the Human Rights Commission from two employees to 35.

[citation needed] In 1970, he was elected chairman of the National Governor's Association which held its annual conference at Lake of the Ozarks.

[4] After facing public pressure to appoint a special counsel during the Watergate scandal, President Richard Nixon considered either Hearnes or Pat Brown for the role.

The winner, Jerry Litton, had 45.4%, but was killed in a plane crash en route to a primary election victory party on August 3.

On August 21, the Democratic state committee selected Hearnes as the replacement nominee over Jim Spainhower by a 38 to 22 vote.

[19] In 1978 he ran unsuccessfully for Missouri state auditor, losing the general election to Republican James F. Antonio, who received 50.8% of the vote.

[20] In 1980, Hearnes was appointed Circuit Court Judge, making him the first person in Missouri history to serve in all three branches of the state government.

[citation needed] In 2010, a bust of Hearnes sculpted by Sabra Tull Meyer[23] was dedicated outside the Mississippi County Courthouse in Charleston, Missouri.

[27] He is also buried with his daughter, Lynn Cooper Hearnes, who was killed in an auto accident on December 31, 2009, only a few months after the death of her father.