Leach had joined the Democratic Party, which through the mid-20th century was dominated in Louisiana by conservative white members. In 1968, Leach was elected to the Louisiana House of Representatives. In his first term, he served in a two-member district with fellow Democrat E. Holman Jones of Oakdale, Allen Parish. Leach was reelected in 1972 and 1976. In 1979, he gave up his seat to run for U.S. Representative from Louisiana's 4th congressional district. In 1980, he was unseated by fellow Democrat Buddy Roemer of Bossier Parish. Roemer later shifted to the Republican Party, a pattern increasingly followed by conservative whites in the state. In 1983, Leach sought to regain his former state legislative seat. He defeated incumbent Democrat, William H. West. In his last term as a state legislator, Leach served on the House Ways and Means Committee. In this capacity, he recommended that a tax be placed on foreign oil processed within the state.
Campaign for Governor
In 2003, Leach launched his gubernatorial campaign, seeking a runoff berth in a crowded field. He campaigned on changing the operations of state government. He suggested that the state use a "brillo pad" to "scrub the budget." Having been diagnosed with polio at a young age, the topic of health care in Louisiana was one of his main priorities. Many young voters were attracted to his campaign's message. He had teams of volunteers set up in Shreveport, Baton Rouge, and New Orleans. Many other candidates were in the race, including Democrats Kathleen Blanco, the lieutenant governor; and Richard Ieyoub, the outgoing state attorney general. Republicans ran Bobby Jindal as their candidate. Leach came in fourth place in the blanket primary behind Kathleen Blanco, Bobby Jindal, the top two finishers, and Richard Ieyoub. Kathleen Blanco went on to win the general election over Republican Jindal.
Chairman of the Louisiana Democratic Party
In 2010, Leach was elected chairman of the Louisiana Democratic Party in a special election. During his tenure as chairman, the party struggled to field candidates for statewide offices. This was a contrast from when Leach first entered politics in what was then an overwhelmingly Democratic state dominated by conservative whites. Before the late 1960s, African Americans were still largely disenfranchised in the state, totally excluded from politics, as they had been since 1898. The party was unable to find a well-funded candidate to run in 2011 against Governor Bobby Jindal. For the first time since Reconstruction, Democrats lost both houses of the state legislature to Republicans. On April 28, 2012, Leach lost his reelection bid as state party chair to State SenatorKaren Carter Peterson of New Orleans.