Thomas Charles O'Halleran is an American politician who served as the U.S. representative for Arizona's 1st congressional district from 2017 to 2023. Beginning his political career as a Republican, he was the Arizona state senator from the 1st district from 2007 to 2009. In 2015, he became a member of the Democratic Party. He also served as the chair of communications for the Blue Dog Coalition. On June 7, 2023, he was appointed as a senior advisor to the U.S. Department of Agriculture in the Joe Biden administration.

Arizona's 1st congressional district is a congressional district located in the U.S. state of Arizona, covering northeastern Maricopa County. Before 2023, geographically, it was the eleventh-largest congressional district in the country and included much of the state outside the Phoenix and Tucson metropolitan areas. From 2013 through 2022, it also included the Navajo Nation, the Hopi reservation, and the Gila River Indian Community, with 25% of the population being Native American. At that time, the district had more Native Americans than any other congressional district in the United States. In the 2022 elections, David Schweikert was elected in the redefined district. It was one of 18 districts that would have voted for Joe Biden in the 2020 presidential election had they existed in their current configuration while being won or held by a Republican in 2022.