When the 1945 death of Franklin Roosevelt propelled Vice President Truman into the presidency, Truman urged placing the Speaker, as an elected representative of his district, as well as the chosen leader of the "elected representatives of the people", next in line to the vice president. Since one could make the same argument for the president pro tempore, Truman's decision may have reflected his strained relations with 78-year-old President Pro Tempore Kenneth McKellar and his warm friendship with 65-year-old House Speaker Sam Rayburn. After all, it was in Rayburn's hideway office, where he had gone for a late afternoon glass of bourbon, that Truman first learned of his own elevation to the presidency.
President Pro Tempore，美国参议院院临时议长，或称美国临时参议长，是美国国会参议院地位第二高的成员和地位最高的参议员。据说杜鲁门把关系更加好的众议院议长的（Speaker）的顺位提到到关系紧张的参议院临时议长（President Pro Tempore）之前。
所以现在（2020年）的顺序是：（Here they are, currently, in order:）
1. Vice President Mike Pence
2. Speaker of the House Nancy Pelosi
3. President Pro Tempore of the Senate Charles Grassley
4. Secretary of State Mike Pompeo
5. Secretary of the Treasury Steve Mnuchin
6. Secretary of Defense Mark Esper
7. Attorney General Bill Barr
14. Secretary of Transportation (However, Elaine Chow, the current officeholder, is ineligible as she is not a "natural-born citizen," as required by the Constitution, so the presidency would go to the official next in line) 由于现任交通部长赵小兰不是土生美国公民，所以要跳过她
How the U.S. Government Is Organized
The Constitution of the United States divides the federal government into three branches to make sure no individual or group will have too much power:
Legislative—Makes laws (Congress, comprised of the House of Representatives and Senate)
Executive—Carries out laws (president, vice president, Cabinet, most federal agencies)
Judicial—Evaluates laws (Supreme Court and other courts)
Each branch of government can change acts of the other branches:
The president can veto legislation created by Congress and nominates heads of federal agencies.
Congress confirms or rejects the president's nominees and can remove the president from office in exceptional circumstances.
The Justices of the Supreme Court, who can overturn unconstitutional laws, are nominated by the president and confirmed by the Senate.
This ability of each branch to respond to the actions of the other branches is called the system of checks and balances.