Detroit Democrat Charles Youngblood was convicted of conspiracy to bribe a public official over a liquor license for a supermarket chain. When his fellow Democrats said they would vote to expel him in 1974, he resigned instead.
Sources:
Robert H. Longstaff, “Youngblood Case Tough for Senate“, Ann Arbor News, January 10, 1974.
“Senator Youngblood Resigns in the Face of Senate Expulsion“, Battle Creek Enquirer, November 17, 1973, p. 8.
Emily Lawler, “Death, Drugs, Skullduggery: A Brief History of Michigan Political Scandals“, MLive, August 21, 2015; updated August 24, 2015.