United States Senate

===Elections and term===
Originally, senators were selected by the [[State legislature (United States)|state legislatures]], not by [[Universal suffrage|popular elections]]. By the early years of the 20th century, the legislatures of as many as 29 states had provided for popular election of senators by referendums.<ref name="direct">{{Cite web |title=Direct Election of Senators |url=https://www.senate.gov/artandhistory/history/common/briefing/Direct_Election_Senators.htm |url-status=live |archive-url=https://web.archive.org/web/20201124193803/https://www.senate.gov/artandhistory/history/common/briefing/Direct_Election_Senators.htm |archive-date=November 24, 2020 |access-date=April 23, 2019 |publisher=U.S. Senate official website}}</ref> Popular election to the Senate was standardized nationally in 1913 by the ratification of the [[Seventeenth Amendment to the United States Constitution|Seventeenth Amendment]].

====Elections====
{{further|List of United States Senate elections}}
Elections to the Senate are held on the first Tuesday after the first Monday in November in even-numbered years, [[Election Day (United States)|Election Day]], and occur simultaneously with elections for the [[United States House of Representatives|House of Representatives]].<ref>{{USC|2|1}}</ref> Senators are elected by their state as a whole. The [[Elections Clause]] of the [[United States Constitution]] grants each state (and Congress, if it so desires to implement a uniform law) the power to legislate a method by which senators are elected. [[Ballot access]] rules for independent and minor party candidates also vary from state to state.

In 45 states, a [[primary election]] is held first for the Republican and Democratic parties (and a select few [[Third party (United States)|third parties]], depending on the state) with the general election following a few months later. In most of these states, the nominee may receive only a plurality, while in some states, a runoff is required if no majority was achieved. In the general election, the winner is the candidate who receives a [[plurality (voting)|plurality]] of the popular vote.

However, in five states, different methods are used. In [[Georgia (U.S. state)|Georgia]], a runoff between the top two candidates occurs if the plurality winner in the general election does not also win a majority. In [[California]], [[Washington (state)|Washington]], and [[Louisiana]], a [[nonpartisan blanket primary]] (also known as a "jungle primary" or "top-two primary") is held in which all candidates participate in a single primary regardless of party affiliation and the top two candidates in terms of votes received at the primary election advance to the general election, where the winner is the candidate with the greater number of votes. In Louisiana, the [[Louisiana primary|blanket primary is considered the general election]] and candidates receiving a majority of the votes is declared the winner, skipping a run-off. In [[Maine]] and [[Alaska]], [[Instant-runoff voting|ranked-choice voting]] is used to nominate and elect candidates for federal offices, including the Senate.<ref>{{Cite web |last=Brooks |first=James |date=December 14, 2020 |title=Election audit confirms win for Ballot Measure 2 and Alaska's new ranked-choice voting system |url=https://www.adn.com/politics/2020/12/14/election-audit-confirms-win-for-ballot-measure-2-and-alaskas-new-ranked-choice-voting-system/ |url-status=live |archive-url=https://web.archive.org/web/20210219222510/https://www.adn.com/politics/2020/12/14/election-audit-confirms-win-for-ballot-measure-2-and-alaskas-new-ranked-choice-voting-system/ |archive-date=February 19, 2021 |access-date=January 10, 2021 |website=Anchorage Daily News}}</ref>

====Vacancies====
The [[Seventeenth Amendment to the United States Constitution|Seventeenth Amendment]] requires that vacancies in the Senate be filled by special election. Whenever a senator must be appointed or elected, the [[secretary of the Senate]] mails one of three forms to the state's governor to inform them of the proper wording to certify the appointment of a new senator.<ref>{{Cite web |title=The Term of A Senator – When Does It Begin and End? – Senate 98-29 |url=https://www.senate.gov/reference/resources/pdf/termofasenator.pdf |url-status=live |archive-url=https://web.archive.org/web/20201222214934/https://www.senate.gov/reference/resources/pdf/termofasenator.pdf |archive-date=December 22, 2020 |access-date=November 13, 2015 |website=United States Senate |publisher=United States Printing Office |pages=14–15}}</ref> If a special election for one seat happens to coincide with a general election for the state's other seat, each seat is contested separately. A senator elected in a special election takes office as soon as possible after the election and serves until the original six-year term expires (i.e. not for a full-term).

The Seventeenth Amendment permits state legislatures to empower their governors to make temporary appointments until the required special election takes place.

The manner by which the Seventeenth Amendment is enacted varies among the states. A 2018 report breaks this down into the following three broad categories (specific procedures vary among the states):<ref name="CRS_R44781">{{Cite web |last=Neale |first=Thomas H. |date=April 12, 2018 |title=U.S. Senate Vacancies: Contemporary Developments and Perspectives |url=https://fas.org/sgp/crs/misc/R44781.pdf |url-status=live |archive-url=https://web.archive.org/web/20180605230617/https://fas.org/sgp/crs/misc/R44781.pdf |archive-date=June 5, 2018 |access-date=October 13, 2018 |website=fas.org |publisher=Congressional Research Service}} '''NOTE:''' wherever present, references to page numbers in superscripts refer to the electronic (.pdf) pagination, not as found printed on the bottom margin of displayed pages.</ref>
* Four states – [[North Dakota]], [[Oregon]], [[Rhode Island]], and [[Wisconsin]] – do not empower their governors to make temporary appointments, relying exclusively on the required special election provision in the Seventeenth Amendment.<ref name="CRS_R44781" />{{rp|7–8}}
* Eight states – [[Alaska]], [[Connecticut]], [[Louisiana]], [[Massachusetts]], [[Mississippi]], [[Texas]], [[Vermont]], and [[Washington (state)|Washington]] – provide for gubernatorial appointments, but also require a special election on an accelerated schedule.<ref name="CRS_R44781" />{{rp|10–11}}
* The remaining thirty-eight states provide for gubernatorial appointments, "with the appointed senator serving the balance of the term or until the next statewide general election".<ref name="CRS_R44781" />{{rp|8–9}}

In ten states within the final category above – [[Arizona]], [[Hawaii]], [[Kentucky]],<ref>{{cite web|url=https://www.courier-journal.com/story/news/politics/ky-general-assembly/2021/03/29/kentucky-lawmakers-nix-veto-mcconnell-backed-senate-vacancy-plan/7055091002/|title=Kentucky lawmakers override veto of Mitch McConnell-backed Senate vacancy plan|author=Morgan Watkins|publisher=Louisville Courier Journal|access-date=September 2, 2023}}</ref> [[Maryland]], [[Montana]], [[North Carolina]], [[Oklahoma]], [[Utah]], [[West Virginia]], and [[Wyoming]] – the governor must appoint someone of the same political party as the previous incumbent.<ref name="CRS_R44781" />{{rp|9}}<ref name=OCPA>{{cite news |title=House approves appointment process for U.S. Senate vacancies |url=https://www.ocpathink.org/post/house-approves-appointment-process-for-u-s-senate-vacancies |work=OCPA |publisher=Oklahoma Council of Public Affairs |date=May 27, 2021 |access-date=December 21, 2022 |archive-date=February 13, 2023 |archive-url=https://web.archive.org/web/20230213095954/https://www.ocpathink.org/post/house-approves-appointment-process-for-u-s-senate-vacancies |url-status=live }}</ref>

In September 2009, Massachusetts changed its law to enable the governor to appoint a temporary replacement for the late senator Edward Kennedy until the special election in January 2010.<ref name="MA Law 54-140">{{Cite web |last=DeLeo |first=Robert A. |date=September 17, 2009 |title=Temporary Appointment of US Senator |url=http://www.mass.gov/legis/laws/mgl/54-140.htm |url-status=live |archive-url=https://web.archive.org/web/20190829050913/https://malegislature.gov/Laws/GeneralLaws/PartI/TitleVIII/Chapter54/Section140 |archive-date=August 29, 2019 |access-date=September 28, 2009 |publisher=Massachusetts Great and General Court}}</ref><ref name="Temporary Appointment Not a Candidate">{{Cite web |last=DeLeo |first=Robert A. |date=September 17, 2009 |title=Temporary Appointment of US Senator Shall not be a candidate in special election |url=https://malegislature.gov/Document/Download?entityTypeName=Journal&generalCourtNumber=186&branchName=2&entityNumber=09-22-09 |url-status=live |archive-url=https://web.archive.org/web/20210108073223/https://malegislature.gov/Document/Download?entityTypeName=Journal&generalCourtNumber=186&branchName=2&entityNumber=09-22-09 |archive-date=January 8, 2021 |access-date=July 19, 2015 |publisher=[[Massachusetts General Court]]}}</ref>

In 2004, Alaska enacted legislation and a separate ballot referendum that took effect on the same day, but that conflicted with each other. The effect of the ballot-approved law is to withhold from the governor authority to appoint a senator.<ref name="Alaska appointments">{{Cite web |date=October 28, 2009 |title=Stevens could keep seat in Senate |url=http://www.adn.com/politics/story/569836.html |url-status=dead |archive-url=https://web.archive.org/web/20090528015846/http://www.adn.com/politics/story/569836.html |archive-date=May 28, 2009 |website=Anchorage Daily News}}</ref> Because the 17th Amendment vests the power to grant that authority to the legislature&nbsp;– not the people or the state generally&nbsp;– it is unclear whether the ballot measure supplants the legislature's statute granting that authority.<ref name="Alaska appointments" /> As a result, it is uncertain whether an Alaska governor may appoint an interim senator to serve until a special election is held to fill the vacancy.

In May 2021, Oklahoma permitted its governor again to appoint a successor who is of the same party as the previous senator for at least the preceding five years when the vacancy arises in an even-numbered year, only after the appointee has taken an oath not to run in either a regular or special Senate election.<ref name=OCPA/>

====Term====
Senators serve terms of six years each; the terms are staggered so that approximately one-third of the seats are up for election every two years. This was achieved by dividing the senators of the [[1st United States Congress|1st Congress]] into thirds (called [[Classes of United States senators|classes]]), where the terms of one-third expired after two years, the terms of another third expired after four, and the terms of the last third expired after six years. This arrangement was also followed after the admission of new states into the union. The staggering of terms has been arranged such that both seats from a given state are not contested in the same general election, except when a vacancy is being filled. [[Class I Senator|Class I]] comprises Senators whose six-year terms are set to expire on January 3, 2025. There is no constitutional [[Term limits in the United States|limit to the number of terms]] a senator may serve.

The Constitution set the date for Congress to convene — Article 1, Section 4, Clause 2, originally set that date for the third day of December. The [[Twentieth Amendment to the United States Constitution|Twentieth Amendment]], however, changed the opening date for sessions to noon on the third day of January, unless they shall by law appoint a different day. The Twentieth Amendment also states that the Congress shall assemble at least once every year, and allows the Congress to determine its convening and adjournment dates and other dates and schedules as it desires. Article 1, Section 3, provides that the president has the power to convene Congress on extraordinary occasions at his discretion.<ref>{{cite web|url=https://www.senate.gov/reference/Sessions/sessionDates.htm|title=Dates of Sessions of the Congress|work=United States Senate|access-date=June 17, 2020|archive-date=December 8, 2020|archive-url=https://web.archive.org/web/20201208023235/https://www.senate.gov/reference/Sessions/sessionDates.htm|url-status=live}}</ref>

A member who has been elected, but not yet seated, is called a ''senator-elect''; a member who has been appointed to a seat, but not yet seated, is called a ''senator-designate''.