Louisiana regularly ranks as one of the poorest and unhealthiest states in the country, but Mr. Cassidy’s biography allowed him to pre-emptively combat suggestions that he was insensitive to the needs of the poor: According to his congressional website, he founded a community health care clinic, treated uninsured patients at the public Earl K. Long Hospital in Baton Rouge for two decades, and set up an emergency medical facility for Katrina evacuees in an abandoned Kmart shortly after the storm.

An early ad featured him in green scrubs and a lab coat, poring over a dog-eared copy of the health care bill by lamplight.

“Most in Congress who voted for Obamacare never read the bill,” he said in the ad, adding: “I read the bill. It was clear there would be canceled plans, expensive premiums, no guarantee you could keep your doctor. I voted no.”

Image Senator Mary L. Landrieu delivered a concession speech on Saturday after losing a runoff election in Louisiana to Bill Cassidy, a Republican. Credit... Jonathan Bachman/Reuters

For Ms. Landrieu, 59, the daughter of former Mayor Moon Landrieu of New Orleans and the sister of the current mayor, Mitch Landrieu, the defeat brings an end to an 18-year run in the Senate in which she earned a reputation as a Democratic Party nonconformist, particularly with her staunch support of the oil and gas industry.

Her prospects in the runoff dimmed after the Nov. 4 election, when Democrats lost their majority in the Senate. That meant that she would no longer chair the Senate Committee on Energy and Natural Resources, which had given her significant sway over the oil and gas industry, which is crucial to Louisiana’s economy.

Senate Republicans also promised Mr. Cassidy a spot on the Energy Committee if he were to win, further undermining Ms. Landrieu’s ability to claim that she was uniquely positioned to protect oil jobs in the state.