The Constitution of Great Britain, which was famously not a thing, defined three entities of what Americans would call government, one elected by the common people, the King or Queen, and in between, the House of Lords, inherited and fancy like the Monarch, but many, and representing the wealth and power of the people.
In a sense, there were three branches of government, the monarchy (king), the aristocracy (we might call them the 1% today), and the democratic branch, aka, the unwashed masses. This conceptualization of the British government is neither new nor mine. In the words of “Massachusettensis,” quoted by John Adams,
“the British constitution consisting of king, lords and commons, is formed upon the principles of monarchy, aristocracy and democracy, in due proportion; that it includes the principled excellencies, and excludes the principal defects of the other kinds of government—the most perfect system that the wisdom of ages has produced, and Englishmen glory in being subject to and protected by it.”
When the Founders famously founded, they created a different set of three branches, but with echoes of the British system. The Senate, of course, is the analog to the House of Lords. The Senate was not meant to be staffed by blue blooded stiffs, but it was meant to be kept separate from the rabble, not elected. Rather, Senators were to be sent to the Federal City (later to be Washington DC) by the states. According to Article I Section 3 of the US Constitution,
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.
The hope was that by linking the Senate to the politics of the states, there would be a more seamless ratification of the Constitution. Lots of stuff was like that in the Constitution; compromises or payoffs of some kind to get the thing ratified or to satisfy some whiney faction or another, while standing against the central principles. This is how things like slavery got through that filter.
This got messy. A state legislature “chooses,” or for that matter, does anything, by passing a law (usually) so “chosen by the Legislature” meant “you figure it out” and from this many systems emerged. If I recall correctly, some states actually had a popular vote for Senate which was then recommended to the Legislature of the state and ignored or not. Some states had a ruling party pick a senator. The point of all this was to have the Senate be staffed by a better class of people as one might expect those already elected to be. The people elect and thus exert a Darwinian process on themselves, and among these, the better survive to elect others. By the time you get to the top you’ve got one fine class of leaders.
What the founders failed to grok is that within a few decades the entire American political system would be taken over by organized crime, and eventually, become the organized crime. With the exception of the occasional true leader, many elected officials were on the take or otherwise not particularly interested in governing. They were, essentially, paid off to keep their hands off the process of industry and commerce, not to represent the people. As for the people themselves, they were not much interested. Little known fact: early voting rates were lower than today’s. Voter apathy is not one of those problems our ancestors did not have and somehow we got. (Voting rates did go up by the end of the 19th century and stayed high for a while in the early 20th century, but this happened as the corruption in government started to become more of a problem, labor got organized, and totally fake leadership like we usually had in the US was being given a rather stern look by the populous. Then they went down again for reasons.)
So, for much of the 19th and early 20th century, the Senate was a mess. It was very common for a state to send only one, or even zero, Senators, for several months or years in a row to DC. Basically, what could have been a Darwinian process of selection for the best became a Darwinian process for of selection for the dirtiest in State governments, and having a Senator in Washington was not a priority at that level. Infighting and incompetence led to lack of representation in the American version of the House of Lords; Those who showed up tended to be those who best handled that ugly process. The British had a House of Lords. We had a House of … something else.
The people of some states rebelled, and forced their states to allow the direct election of Senators. After a while, there were enough of these Senators in the Senate, and thus favorable to this idea, to create a draft Constitutional Amendment. This was eventually supported by the House as well, voted on, and over a period of a few years, ratified. A process that started in the late 19th century finally culminated in 1913 with the 17th Amendment. This change happened:
The Senate of the United States shall be composed of two Senators from each State,
chosen by the Legislature thereofelected by the people thereof, for six years; and each Senator shall have one vote….
So, that was interesting, but what happens when a Senator dies in office or leaves for some other reason?
The 17th Amendment goes on to say,
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.
Once again, the US Constitution trusts states to do the right thing. Oh boy.
For the most part, in most states, if a Senate seat is suddenly vacant, the governor of that state appoints a replacement until either a special election is held, or the end of the term. Usually, there is a special election, but if the vacancy occurs after the typical November election before the normal expiration of the term, most governors will not bother with the election, and the appointed Senator simply holds office until the normal election.
In Wyoming and Utah, the governor is required to pick a temporary senator from a list of three put forth by the political party held by the Senator who left office. Alaska, Oregon, and Wisconsin require the special election and does not allow the governor to appoint. Oklahoma either calls for the special election or a governor’s temporary appointment, depending on the timing of the vacancy.
Hawaii and Arizona require the governor to appoint a temporary Senator from the party that the Senator who left office was from. So, for example, if Senator Mazie Hirono were to get a new job, the governor of Hawaii would appoint a Democrat to fill her spot. If a Senator from Arizona flaked out, the governor of Arizona would appoint a Republican, as both Senators from Arizona are Republicans. So there would not be a change in representation by party in the US Senate.