In 1945, President Truman "proposed that Congress revise the order of succession, placing the Speaker of the House and the President Pro Tempore of the Senate in line behind the Vice President and ahead of the cabinet," wrote Thomas H. Neale in the monograph "Presidential and Vice Presidential Succession: Overview and Current Legislation" (2003). "Truman argued that it was more appropriate and democratic to have popularly elected officials first in line to succeed, rather than appointed cabinet officers. Although Truman’s proposal also provided for special elections to fill simultaneous vacancies in the presidency and vice presidency, Congress passed only its succession aspects in the Presidential Succession Act of 1947 (61 Stat. 380).
"Under the Act, if both the presidency and vice presidency are vacant, the Speaker succeeds (after resigning the speakership and his House seat). If there is no Speaker, or if he does not qualify, the President Pro Tempore succeeds, under the same requirements. If there is neither a Speaker nor President Pro Tempore, or if neither qualifies, then the cabinet officers succeed, under the same conditions as applied in the 1886 act. The Presidential Succession Act of 1947 has been regularly amended to incorporate new cabinet-level departments into the line of succession, and remains currently in force."