Lucas was born on a tenant farm near Chandlerville, in Cass County, Illinois. He was the youngest of six children of William Douglas and Sarah Catherine Lucas. His parents named him after Scott Wike, a Democrat who served a representative from Illinois. After attending public schools, he began his studies at Illinois Wesleyan University. During college, he was active in athletics. He lettered in football, basketball, and baseball and played semiprofessional baseball in the Three-I League during his summer breaks. Lucas graduated from Wesleyan with a law degree in 1914 and was admitted to the bar the following year. He served as a schoolteacher before entering private practice in Havana. During World War I, he served in the US Army and rose to become a lieutenant. Lucas returned to his law practice following his military service and served as a state's attorney for Mason County from 1920 to 1925. He also worked as a commander of the Illinois Department of the American Legion. In 1932, he was defeated by William H. Dieterich for the Democratic nomination to challenge Republican incumbent Otis F. Glenn for a United States Senate seat from Illinois. Lucas was later appointed chairman of State Tax Commission by GovernorHenry Horner, serving from 1933 to 1935.
In 1938, after William Dieterich declined to run for re-election, Lucas was elected to the U.S. Senate over Republican Richard J. Lyons, with a 51%-48% victory. He was re-elected in 1944. Lucas was a favorite son candidate and among twelve nominated at the 1944 Democratic National Convention to serve as Franklin D. Roosevelt's running mate in the presidential election that year. With support from Harry S. Truman, he was elected party whip in 1946. Lucas, a moderate, drew support from both conservative and liberal wings of the party. He took over the Midwest campaign for Truman and was credited with assisting Truman's 1948 re-election and bringing nine Democrats into the Senate. When Alben Barkley became vice-president and resigned his seat, Lucas became majority leader. However, he was unable to build a consensus as Senate Majority Leader with the onset of the anticommunist era, and lost in 1950, to Republican Everett Dirksen. Lucas had become a target of Republican wrath with loss of political power in the Senate and the White House. His 1950 reelection campaign featured the active intervention into Illinois politics of Wisconsin Senator Joseph McCarthy, who traveled the state with Dirksen saying that Senator Lucas was "soft on communism." Dirksen would go on to decisively defeat Lucas with a 54% to 46% victory. Privately, in later years, Dirksen attributed his victory to Lucas's responsibilities as Senate Majority Leader, at the apparent expense of his state; Dirksen was free to campaign locally, often debating Lucas's Illinois Democratic Party proxies and calling attention to Lucas's prolonged absence from the state.
