 The most notorious of Montana's early Senators was William A. Clark, a "Copper King" and one of the 50 richest Americans ever.He is well known for having bribed his way into the U.S. Senate.Among Montana's most historically prominent Senators are Thomas J. Walsh (serving from 1913 to 1933), who was President-elect Franklin D. Roosevelt's choice for Attorney General when he died; Burton K. Wheeler (serving from 1923 to 1947), an oft-mentioned presidential candidate and strong supporter of isolationism; Mike Mansfield, the longest-serving Senate Majority Leader in U.S. history; Max Baucus (served 1978 to 2014), longest-serving U.S.