Who chooses the majority party?

"Elected at the beginning of each Congress by members of their respective party conferences to represent them on the Senate floor, the majority and minority leaders serve as spokesmen for their parties' positions on the issues. The majority leader has also come to speak for the Senate as an institution. Working with the committee chairs and ranking members, the majority leader schedules business on the floor by calling bills from the calendar and keeps members of his party advised about the daily legislative program. In consultation with the minority leader, the majority leader fashions unanimous consent agreements by which the Senate limits the amount of time for debate and divides that time between the parties. When time limits cannot be agreed on, the majority leader might file for cloture to shut off debate. Occupying the front desks on the center aisle, the two leaders coordinate party strategy and try to keep their parties united on roll-call votes." (From "Majority and Minority Leaders and Party Whip," http://www.senate.gov/artandhistory/history/common/briefing/Majority_Minority_Leaders.htm)