Bailey dominated Connecticut Democratic politics as a party chairman, from 1946 to his death in 1975. He typically had a decisive voice in selecting the party's candidates for top offices and in coordinating Democrats in the state legislature. He was even more powerful as the chairman of the Democratic National Committee from 1961 until 1968, and was one of the main behind-the-scenes backers of John F. Kennedy.
Bailey was the dominant figure in Connecticut politics between 1950 and his death in 1975. From his office in Hartford two blocks from the State Capitol, he co-ordinated and controlled statewide election campaigns and the activities of the Connecticut General Assembly.
Bailey's tenure as head of the Connecticut Democratic party was credited with turning the state from one politically dominated by WASP Yankee Republicans to one dominated by Democratic candidates of Roman Catholic and Jewish background, such as Abraham Ribicoff, Thomas Dodd, John Dempsey, and Ella T. Grasso.
His Republican rival was Meade Alcorn, who also happened to serve as national chairman of his party.
Prior to his statewide activities, Bailey had served in local roles in his home city of Hartford.
Bailey's term as DNC chairman was a roller-coaster ride, as he oversaw the party's moment of greatest political strength (following the 1964 electoral landslide) and greatest political weakness (the 1968 Democratic National Convention in Chicago).
Following the assassination of John F. Kennedy in 1963, Bailey and the new president, Lyndon B. Johnson, oversaw the greatest electoral landslide in United States history, with the party winning 486 electoral votes as well as supermajorities in both houses of the US Congress. The 1966 elections to the House of Representatives and the Senate saw Republican gains but Democrats retaining control of both houses of Congress as well as the majority of governorships.
In 1968, the Republican Party again nominated Richard Nixon as the presidential nominee and quickly rallied around him. However, the Democrats were more divided, particularly over the controversial Vietnam War. Senator Eugene McCarthy, Senator Robert F. Kennedy, Senator George McGovern, Vice President Hubert Humphrey were only some of those who sought the nomination, with Bailey co-presiding over the 1968 Democratic National Convention in Chicago. In the end, Nixon was elected, but the Democrats retained their majority in both houses of Congress.
The John M. Bailey Papers have been donated to the University of Connecticut and are available for research.
The Bailey and Kennelly families have a prominent presence in the Connecticut Democratic Party and in their home city of Hartford. In Hartford, they're one of the last prominent Irish families in a city that is now dominated by African-Americans and Puerto Ricans. 
Bailey's daughter, Barbara Bailey Kennelly served in the U.S. Congress representing Connecticut's first congressional district. She later left the House to run for governor in a race she lost to the incumbent, John G. Rowland.
Another Bailey daughter, Judith Bailey Perkins, is a professor at Saint Joseph College in West Hartford.
Bailey's son, also named John Bailey, was a career prosecutor in Connecticut, first serving as Hartford State's Attorney, then as Chief State's Attorney.
Many of Bailey's grandsons continue his legacy of public and political service: John Moran Bailey II is currently the Director of Government and Community Affairs for a nonprofit, SINA Inc., working to stabilize one of Hartford's poorest and blighted communities; another grandson Justin Kronholm, is the former Executive Director of the Connecticut Democratic State Central Committee.
Another grandson, John B. Kennelly was previously elected to Hartford Court of Common Council and was an early rival to Hartford mayor Eddie Perez.