A curious thing happened this past presidential cycle.
Seems that the "Big Three" were all sitting senators.
So....what happens when there's a senate vacancy? The constitution is actually fairly gray in that regard. Article 1 gives a long enumeration of who can be a senator, and what their duties are, but includes the following caveat:
The Senate of the United States shall be composed of two Senators from each state, chosen by the legislature thereof, for six years; and each Senator shall have one vote.
So in the early days of the Republic, our senators were chosen by the state legislature. [Representatives were always selected by general election.]
But somehere along the way, Congress decided to ammend the procedure, and the result was the 17th Ammendment (circa 1913):
The Senate of the United States shall be composed of two Senators from each state, elected by the people thereof, for six years; and each Senator shall have one vote. The electors in each state shall have the qualifications requisite for electors of the most numerous branch of the state legislatures.
When vacancies happen in the representation of any state in the Senate, the executive authority of such state shall issue writs of election to fill such vacancies: Provided, that the legislature of any state may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct.
This amendment shall not be so construed as to affect the election or term of any Senator chosen before it becomes valid as part of the Constitution.
So what was initially a gray process became grayer still, as the Constitution now directs that the governor shall appoint a replacement unless the state legislator overturns the local authority and calls for a special election.
What this all means is that like many things in these United States...it's inconsistent. Using a local example, the Governor of this Commonwealth was empowered to appoint a replacement senator, and always had been.
Until 2004. That year, one of our senators almost
became president. At the time, we had a Republican governor, and the state was paralyzed by the fear that he would appoint a Republican replacement. The state hurriedly passed legislation calling for a special election, and that's what we'll do the next time a Massachusetts Senator is forced to leave office before his term expires.