John Fitzgerald "Jack" Kennedy, often referred to by his initials JFK, was the 35th President of the United States, serving from 1961 until his assassination in 1963. After military service as commander of the Motor Torpedo Boat PT-109 and Motor Torpedo Boat PT-59 during World War II in the South Pacific, Kennedy represented Massachusetts's 11th congressional district in the U.S. House of Representatives from 1947 to 1953 as a Democrat. Thereafter, he served in the U.S. Senate from 1953 until 1960. Kennedy defeated then Vice President and Republican candidate Richard Nixon in the 1960 U.S. presidential election. He was the youngest elected to the office, at the age of 43, the second-youngest President (after Theodore Roosevelt), and the first president to have been born in the 20th century.