The correct answer is: "the President Pro Tempore".
The President pro tempore of the US Senate is its leader in absence of the Vice President who, according to the US Consittution, is the President of the Senate. Therefore he operates regularly as the president of the chamber, substituting the Vice President.
The President pro tempore is a senior member of the party that occupies the largest proportion of seats in the Senate, and therefore is allowed to vote and to speak on any issue.