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William Morgan Cassidy (born September 28, 1957) is an American physician and politician serving as the senior United States senator from Louisiana, a seat he has held since 2015. A member of the Republican Party, he served in the Louisiana State Senate from 2006 to 2009 and in the U.S. House of Representatives from 2009 to 2015. Cassidy chairs the Senate Health, Education, Labor, & Pensions (HELP) Committee.

Born in Highland Park, Illinois, Cassidy is a graduate of Louisiana State University (LSU) and LSU School of Medicine. A gastroenterologist, he was elected to the Louisiana State Senate from the 16th district which included parts of Baton Rouge, in 2006. In 2008, he was elected as the U.S. representative for Louisiana's 6th congressional district, defeating Democratic incumbent Don Cazayoux. Cassidy was elected to the Senate in 2014, defeating Democratic incumbent Mary Landrieu. He was reelected in 2020.

A moderate Republican, Cassidy is an outspoken critic of President Donald Trump. In 2021, he was one of seven Republican senators to vote to convict Trump of incitement of insurrection in his second impeachment trial. As a result, the Republican Party of Louisiana censured him. In 2023, after Trump was indicted for mishandling classified documents, Cassidy called for Trump to drop out of the 2024 presidential election. After Trump secured the Republican nomination, Cassidy declined to endorse him in the general election.