the president pro temp
The 'President Pro Tempore' is the President of the Senate, when the Vice President of the United States is absent.
The U.S. Vice President is the President of the Senate. When he/she is absent or acting as President, the Senate President Pro Tempore is in charge.
The president pro tempore presides over the Senate when the Vice President is absent. The current president pro tempore (as of 5/17) is Senator Orrin Hatch of Utah.
The Vice President of the United States is in charge of the U.S. Senate.
When the US Vice President is absent or acting as President, his/her Senate duties fall to the US Senate President Pro Tempore, who by tradition is the longest-serving Senator of the majority party.
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President Pro-Tempore
If the vice-president is absent, some other senator, such are the pro tempore, presides over the Senate.
Because
The president pro tempore serves as the head of the senate in place of the vice president- APEX
The Leader of the Senate is always the Vice-President. And if the Vice-President isn't there, then the President pro-tempore(temporary President) is in charge.
As far as the progression of power it goes President Vice President and the Speaker of the House of Representatives.