Ridge joins other members of Bush's Cabinet Commerce Secretary Don Evans, Education Secretary Rod Paige, Agriculture Secretary Ann Veneman, Energy Secretary Spencer Abraham, Attorney General John Ashcroft and Secretary of State Colin Powell who decided not to serve the president during his second term.
Ridge took office in January 2003 as the first secretary of the Department of Homeland Security, which became the catalyst for the largest reorganization of the federal government since the Department of Defense was created following WW II.
Ridge reportedly told colleagues he might resign after the November election to enter the private sector.
No replacement has been named. All nominees for Cabinet positions must be confirmed by the Senate.