While serving the House, McCollum was also selected for a variety of Republican leadership positions, including three terms as Vice Chairman of the House Republican Conference. McCollum gained national attention as one of 15 members selected to serve on the House Committee to Investigate the Iran-Contra Affair, and, in 1998–1999, as one of the House Managers of President Bill Clinton's impeachment trial. Rather than seek reelection to the House in 2000, McCollum ran unsuccessfully for an open United States Senate seat, bringing to an end his 20-year Congressional career. McCollum ran again in 2004, seeking the Republican nomination for the U.S. Senate seat being vacated by retiring Senator Bob Graham. He was defeated in the Republican primary by HUD Secretary Mel Martinez, who went on to win the seat. McCollum served as a partner with the Baker & Hostetler LLP law firm, practicing in the federal policy area. In addition to his duties as the state's chief legal officer, he serves as President and Chairman of the Healthy Florida Foundation, chartered in 2002 to find consensus on long-term solutions to the nation's health care system. He is a member of the North Florida Committee on Foreign Relations. He is also a board member of the James Madison Institute. In 2006, McCollum ran for the office of Attorney General of the State of Florida, defeating State Senator Skip Campbell in the general election. McCollum led a group of Attorneys General in filing a lawsuit challenging the constitutionality of Obamacare. He also filed a brief in support of Arizona's immigration law. McCollum opposed the federal Stimulus bill, as well as the $20 billion federal oil fund that limited his office's ability to pursue claims against BP, and requested additional authority from the federal government to address Medicaid fraud. While Attorney General, McCollum defended Florida's ban of adoptions by homosexuals from a lawsuit, "" challenging the ban. McCollum hired Dr. George Rekers, a controversial clinical psychologist, to testify during the trial that heterosexual parents provide a better environment for children. The trial resulted in the overturning of the ban. When an appellate court upheld the lower court ruling, McCollum declined to appeal the case to the Supreme Court. He was also the Florida Chairman for Rudy Giuliani presidential campaign in 2008. On May 18, 2009 McCollum announced his candidacy for Governor of Florida. The election determined the successor of Charlie Crist who later lost his bid for a seat in the U.S. Senate. McCollum opposed federal health care mandates in Florida, decrying them as an unconstitutional "tax on living," and joined with 13 other state attorneys general in filing a federal lawsuit.