Answer:
The leader of the Senate, or President of the Senate, is usually the Vice President of the United States, and can only be replaced temporarily by a Pro-Tempore President, if, and when, the Vice President is unable to carry out his/her duties. Currently, that leader is Mike Pence, as he was elected since January 20th, 2017 as Vice President to President Donald Trump, and so, he directly became also the President of the Senate. The Pro-Tempore President, nowadays, is Chuck Grassley, who was also elected on January 3rd, 2019.
Explanation:
The position of the President of the Senate, being taken by the Vice President of the United States, has been established by the U.S., Constitution.
