A Democrat, Bentsen was elected Hildago county judge and served in that role from 1946 to 1948. In 1948 Bentsen was elected to serve in the U.S. House of Representatives, where he was a protégé of Speaker of the House Sam Rayburn. In 1955 Bentsen stood down from elective politics and moved his family to Houston, where he worked in the financial industry and solidified his financial position. During this time he founded Consolidated American Life Insurance Company. By the late 1960s he was chairman of Lincoln Consolidated Inc., a financial holdings company. While Bentsen was not seeking office during these years, he remained in touch with Democratic Party politics.
In 1970 Bentsen decided to reenter politics, this time as a candidate for the United States Senate. He won an upset victory over incumbent U.S. Sen. Ralph Yarborough in the Democratic primary and then went on to win the general election over the Republican nominee, U.S. Rep. George H.W. Bush. Bentsen was reelected to the Senate in 1976, 1982, and 1988, eventually serving as chairman of the Senate Finance Committee. He campaigned for the Democratic presidential nomination in 1976 but lost. In the Senate, he was known for his pro-business stance and was a supporter of the oil and gas industry, free trade, and the real estate industry. In 1988 Massachusetts Gov. Michael Dukakis won the Democratic Party nomination for the presidency. Dukakis chose Bentsen to be his vice presidential running mate in the general election. Thanks to Texas election law, Bentsen was able to seek both the vice presidency and his Senate seat, which was up for reelection, that year. Bentsen was easily reelected to his Senate seat. However, the Republican ticket of Vice President George H.W. Bush and U.S. Sen. Dan Quayle of Indiana won the presidential election. Despite the loss of the Dukakis–Bentsen ticket, Bentsen received notoriety for his performance in the nationally-broadcast vice presidential debate. When Quayle compared his political experience to that of President John F. Kennedy, a Democrat, Bentsen replied, "Senator, I served with Jack Kennedy, I knew Jack Kennedy, Jack Kennedy was a friend of mine. Senator, you are no Jack Kennedy."
In 1992 Bentsen was urged to seek the presidency but chose not to make the race. The Democratic nominee, Arkansas Gov. Bill Clinton, won the election. Clinton asked Bentsen to serve as Secretary of the Treasury. The Senate confirmed Bentsen to that post, and he resigned his Senate seat. As secretary, Bentsen played an important role in the formation of the Clinton administration's early fiscal policies. Bentsen served as secretary from 1993 to 1994, and left, he said, because he had planned to retire from politics in 1994, upon the conclusion of what would have been his fourth Senate term. Clinton recalled Bentsen as a "conservative Democrat, a fiscal conservative who thought more prosperous people like him should pay taxes so that those who were less fortunate should be able to get a good education and have some opportunities in life." In 1999 he received the Presidential Medal of Freedom.
Bentsen, who had suffered a stroke in 1998, died in Houston on May 23, 2006, at the age of eighty-five. He was survived by his wife, two sons, a daughter, two brothers, a sister, and seven grandchildren. He was a Presbyterian. Bentsen was buried at Forest Park Lawndale Cemetery in Houston.
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Forest Park Lawndale Cemetery