In 1974, Hart ran for the United States Senate, challenging two-term incumbent Republican Peter Dominick. Hart was aided by Colorado's trend toward Democrats during the early 1970s, as well as Dominick's continued support for the unpopular President Richard Nixon and concerns about the senator's age and health. In the general election, Hart won by a wide margin (57.2% to Dominick's 39.5%) and was immediately labeled a rising star. He got a seat on the Armed Services Committee, and was an early supporter of reforming the bidding for military contracts, as well as an advocate for the military using smaller, more mobile weapons and equipment, as opposed to the traditional large-scale items. He also served on the Environment and Public Work Committee and the Senate Intelligence Committee. From 1975 to 1976, Hart was a member of the post-Watergate Church Committee that investigated abuses by the Central Intelligence Agency, National Security Agency, Federal Bureau of Investigation and the Internal Revenue Service. Hart served as the chairman of Senate Subcommittee on Nuclear Regulation. He flew over the Three Mile Island nuclear reactor near Harrisburg, Pennsylvania, in an Army helicopter several times with fellow Senator Alan Simpson during the nuclear accident and led the subsequent Senate investigation into the incident.