Shannon began to practice law in Kansas City. Shannon and the Pendergasts were all members of the
Democratic Party but frequently clashed in local elections, either directly or through supported candidates on issues such as
Prohibition, the influence of saloon-owners in local politics, and the
Democratic Party's nominee for Kansas City's mayoral and
alderman races. The "rabbits" and the "goats" shared control over the Kansas City Democrats in an agreement they called "Fifty-Fifty": each faction would nominate half of the Democratic candidates for local offices. If there was disagreement between the factions as to whose candidate should run in an office, they would each nominate a candidate for the
primary election and then respect the results of the vote. This scheme allowed the "goats" and "rabbits" to maintain relatively equal control over the Kansas City Democrats until Shannon's faction was relegated to a minor role after Pendergast successfully urged his followers to vote against Democratic candidates that were not backed by his
political machine in the 1916 Kansas City local elections; going as far as openly supporting
Nonpartisan candidates for alderman and council positions. Pendergast-backed candidates won five
city council seats, with Republicans winning the remainder. This result confirmed that Pendergast's faction had control over the party's future. Shannon was chairman of the Democratic State Convention in 1910; delegate to the
Democratic National Conventions in 1908, 1912, 1920, 1924, 1928, 1932, and 1940; and member of the Missouri Constitutional Conventions in 1922 and 1923. He was elected as a
Democrat to the
U.S. House of Representatives, serving from March 4, 1931, to January 3, 1943. Shannon was known as a scholar of
Thomas Jefferson and would speak about his life and his teachings. ==Personal life==